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Title: The Structure and Distribution of Coral Reefs
Author: Charles Darwin
Release Date: June, 2000 [eBook #2690]
[Most recently updated: October 10, 2022]
Language: English
Character set encoding: UTF-8
Produced by: Sue Asscher and David Widger
*** START OF THE PROJECT GUTENBERG EBOOK CORAL REEFS ***
CORAL REEFS
By Charles Darwin
CONTENTS
DETAILED TABLE OF CONTENTS
DESCRIPTION OF THE PLATES
THE STRUCTURE AND DISTRIBUTION OF CORAL REEFS
INTRODUCTION
CHAPTER I.—ATOLLS OR LAGOON-ISLANDS
CHAPTER II.—BARRIER REEFS
CHAPTER III.—FRINGING OR SHORE-REEFS
CHAPTER IV.—ON THE DISTRIBUTION AND GROWTH OF CORAL-REEFS
CHAPTER V.—THEORY OF THE FORMATION OF THE DIFFERENT CLASSES OF
CORAL-REEFS
CHAPTER VI.—ON THE DISTRIBUTION OF CORAL-REEFS WITH REFERENCE TO THE
THEORY OF THEIR FORMATION
APPENDIX
INDEX
DETAILED TABLE OF CONTENTS
DESCRIPTION OF THE PLATES
INTRODUCTION
CHAPTER I.—ATOLLS OR LAGOON-ISLANDS
SECTION I.—DESCRIPTION OF KEELING ATOLL. Corals on the outer
margin.—Zone of Nulliporæ.—Exterior
reef.—Islets.—Coral-conglomerate.—Lagoon.—Calcareous sediment.—Scari
and Holuthuriæ subsisting on corals.—Changes in the condition of the
reefs and islets.—Probable subsidence of the atoll.—Future state of the
lagoon.
SECTION II.—GENERAL DESCRIPTION OF ATOLLS. General form and size of
atolls, their reefs and islets.—External slope.— Zone of
Nulliporæ.—Conglomerate.—Depth of lagoons.—Sediment.—Reefs submerged
wholly or in part.—Breaches in the reef.—Ledge-formed shores round
certain lagoons.—Conversion of lagoons into land.
SECTION III.—ATOLLS OF THE MALDIVA ARCHIPELAGO—GREAT CHAGOS BANK.
Maldiva Archipelago.—Ring-formed reefs, marginal and central.—Great
depths in the lagoons of the southern atolls.—Reefs in the lagoons all
rising to the surface.—Position of islets and breaches in the reefs,
with respect to the prevalent winds and action of the
waves.—Destruction of islets.—Connection in the position and submarine
foundation of distinct atolls.—The apparent disseverment of large
atolls.—The Great Chagos Bank.—Its submerged condition and
extraordinary structure.
CHAPTER II.—BARRIER REEFS
Closely resemble in general form and structure atoll-reefs.—Width and
depth of the lagoon-channels.—Breaches through the reef in front of
valleys, and generally on the leeward side.—Checks to the filling up of
the lagoon-channels.—Size and constitution of the encircled islands.—
Number of islands within the same reef.—Barrier-reefs of New Caledonia
and Australia.—Position of the reef relative to the slope of the
adjoining land.—Probable great thickness of barrier-reefs.
CHAPTER III.—FRINGING OR SHORE-REEFS
Reefs of Mauritius.—Shallow channel within the reef.—Its slow filling
up.—Currents of water formed within it.—Upraised reefs.—Narrow
fringing-reefs in deep seas.—Reefs on the coast of E. Africa and of
Brazil.—Fringing-reefs in very shallow seas, round banks of sediment
and on worn-down islands.—Fringing-reefs affected by currents of the
sea. —Coral coating the bottom of the sea, but not forming reefs.
CHAPTER IV.—ON THE DISTRIBUTION AND GROWTH OF CORAL-REEFS
SECTION I.—ON THE DISTRIBUTION OF CORAL-REEFS, AND ON THE CONDITIONS
FAVOURABLE TO THEIR INCREASE.
SECTION II.—ON THE RATE OF GROWTH OF CORAL-REEFS.
SECTION III.—ON THE DEPTHS AT WHICH REEF-BUILDING POLYPIFERS CAN LIVE.
CHAPTER V.—THEORY OF THE FORMATION OF THE DIFFERENT CLASSES OF
CORAL-REEFS
The atolls of the larger archipelagoes are not formed on submerged
craters, or on banks of sediment.—Immense areas interspersed with
atolls.—Recent changes in their state.—The origin of barrier-reefs and
of atolls.—Their relative forms.—The step-formed ledges and walls round
the shores of some lagoons.—The ring-formed reefs of the Maldiva
atolls.—The submerged condition of parts or of the whole of some
annular reefs.—The disseverment of large atolls.—The union of atolls by
linear reefs.—The Great Chagos Bank.—Objections, from the area and
amount of subsidence required by the theory, considered.—The probable
composition of the lower parts of atolls.
CHAPTER VI.—ON THE DISTRIBUTION OF CORAL-REEFS WITH REFERENCE TO THE
THEORY OF THEIR FORMATION
Description of the coloured map.—Proximity of atolls and
barrier-reefs.— Relation in form and position of atolls with ordinary
islands.—Direct evidence of subsidence difficult to be detected.—Proofs
of recent elevation where fringing-reefs occur.—Oscillations of
level.—Absence of active volcanoes in the areas of
subsidence.—Immensity of the areas which have been elevated and have
subsided.—Their relation to the present distribution of the land.—Areas
of subsidence elongated, their intersection and alternation with those
of elevation.—Amount and slow rate of the subsidence.—Recapitulation.
APPENDIX
INDEX
DESCRIPTION OF THE PLATES.
PLATE I.
Illustration: PLATE I.—MAP SHOWING THE RESEMBLANCE IN FORM BETWEEN
BARRIER CORAL-REEFS SURROUNDING MOUNTAINOUS ISLANDS, AND ATOLLS OR
LAGOON ISLANDS.
In the several original surveys, from which the small plans on this
plate have been reduced, the coral-reefs are engraved in very different
styles. For the sake of uniformity, I have adopted the style used in
the charts of the Chagos Archipelago, published by the East Indian
Company, from the survey by Captain Moresby and Lieutenant Powell. The
surface of the reef, which dries at low water, is represented by a
surface with small crosses: the coral-islets on the reef are marked by
small linear spaces, on which a few cocoa-nut trees, out of all
proportion too large, have been introduced for the sake of clearness.
The entire _annular reef_, which when surrounding an open expanse of
water, forms an “atoll,” and when surrounding one or more high islands,
forms an encircling “barrier-reef,” has a nearly uniform structure. The
reefs in some of the original surveys are represented merely by a
single line with crosses, so that their breadth is not given; I have
had such reefs engraved of the width usually attained by coral-reefs. I
have not thought it worth while to introduce all those small and very
numerous reefs, which occur within the lagoons of most atolls and
within the lagoon-channels of most barrier-reefs, and which stand
either isolated, or are attached to the shores of the reef or land. At
Peros Banhos none of the lagoon-reefs rise to the surface of the water;
a few of them have been introduced, and are marked by plain dotted
circles. A few of the deepest soundings are laid down within each reef;
they are in fathoms, of six English feet.
Figure 1.—VANIKORO, situated in the western part of the South Pacific;
taken from the survey by Captain D’Urville in the _Astrolabe;_ the
soundings on the southern side of the island, namely, from thirty to
forty fathoms, are given from the voyage of the Chev. Dillon; the other
soundings are laid down from the survey by D’Urville; height of the
summit of the island is 3,032 feet. The principal small detached reefs
within the lagoon-channel have in this instance been represented. The
southern shore of the island is narrowly fringed by a reef: if the
engraver had carried this reef entirely round both islands, this figure
would have served (by leaving out in imagination the barrier-reef) as a
good specimen of an abruptly-sided island, surrounded by a reef of the
fringing class.
Figure 2.—HOGOLEU, or ROUG, in the Caroline Archipelago; taken from the
atlas of the voyage of the _Astrolabe_, compiled from the surveys of
Captains Duperrey and D’Urville; the depth of the immense lagoon-like
space within the reef is not known.
Figure 3.—RAIATEA, in the Society Archipelago; from the map given in
the quarto edition of “Cook’s First Voyage;” it is probably not
accurate.
Figure 4.—BOW, or HEYOU ATOLL (or lagoon-island), in the Low
Archipelago, from the survey by Captain Beechey, R.N.; the lagoon is
choked up with reefs, but the average greatest depth of about twenty
fathoms, is given from the published account of the voyage.
Figure 5.—BOLABOLA, in the Society Archipelago, from the survey of
Captain Duperrey in the _Coquille:_ the soundings in this and the
following figures have been altered from French feet to English
fathoms; height of highest point of the island 4,026 feet.
Figure 6.—MAURUA, in the Society Archipelago; from the survey by
Captain Duperrey in the _Coquille:_ height of land about eight hundred
feet.
Figure 7.—POUYNIPÈTE, or SENIAVINE, in the Caroline Archipelago; from
the survey by Admiral Lutké.
Figure 8.—GAMBIER ISLANDS, in the southern part of the Low Archipelago;
from the survey by Captain Beechey; height of highest island, 1,246
feet; the islands are surrounded by extensive and irregular reefs; the
reef on the southern side is submerged.
Figure 9.—PEROS BANHOS ATOLL (or lagoon-island), in the Chagos group in
the Indian Ocean; from the survey by Captain Moresby and Lieutenant
Powell; not nearly all the small submerged reefs in the lagoon are
represented; the annular reef on the southern side is submerged.
Figure 10.—KEELING, or COCOS ATOLL (or lagoon-island), in the Indian
Ocean; from the survey by Captain Fitzroy; the lagoon south of the
dotted line is very shallow, and is left almost bare at low water; the
part north of the line is choked up with irregular reefs. The annular
reef on the north-west side is broken, and blends into a shoal
sandbank, on which the sea breaks.
PLATE II.—GREAT CHAGOS BANK, NEW CALEDONIA, MENCHIKOFF ATOLL, ETC.
Illustration: Plate 2
Figure 1.—GREAT CHAGOS BANK, in the Indian Ocean; taken from the survey
by Captain Moresby and Lieutenant Powell; the parts which are shaded,
with the exception of two or three islets on the western and northern
sides, do not rise to the surface, but are submerged from four to ten
fathoms; the banks bounded by the dotted lines lie from fifteen to
twenty fathoms beneath the surface, and are formed of sand; the central
space is of mud, and from thirty to fifty fathoms deep.
Figure 2.—A vertical section, on the same scale, in an eastern and
western line across the Great Chagos Bank, given for the sake of
exhibiting more clearly its structure.
Figure 3.—MENCHIKOFF ATOLL (or lagoon-island), in the Marshall
Archipelago, Northern Pacific Ocean; from Krusenstern’s “Atlas of the
Pacific;” originally surveyed by Captain Hagemeister; the depth within
the lagoons is unknown.
Figure 4.—MAHLOS MAHDOO ATOLL, together with Horsburgh atoll, in the
Maldiva Archipelago; from the survey by Captain Moresby and Lieutenant
Powell; the white spaces in the middle of the separate small reefs,
both on the margin and in the middle part, are meant to represent
little lagoons; but it was found not possible to distinguish them
clearly from the small islets, which have been formed on these same
small reefs; many of the smaller reefs could not be introduced; the
nautical mark (dot over a dash) over the figures 250 and 200, between
Mahlos Mahdoo and Horsburgh atoll and Powell’s island, signifies that
soundings were not obtained at these depths.
Figure 5.—NEW CALEDONIA, in the western part of the Pacific; from
Krusenstern’s “Atlas,” compiled from several surveys; I have slightly
altered the northern point of the reef, in accordance with the “Atlas
of the Voyage of the _Astrolabe_.” In Krusenstern’s “Atlas,” the reef
is represented by a single line with crosses; I have for the sake of
uniformity added an interior line.
Figure 6.—MALDIVA ARCHIPELAGO, in the Indian Ocean; from the survey by
Captain Moresby and Lieutenant Powell.
PLATE III.—MAP SHOWING THE DISTRIBUTION OF CORAL-REEFS AND ACTIVE
VOLCANOES.
Illustration: Plate 3 Shewing the Distribution of the Different Kinds
of Coral Reefs, Together with the Position of the Active Volcanoes in
the Map.
The principles, on which this map was coloured, are explained in the
beginning of Chapter VI.; and the authorities for each particular spot
are detailed in the Appendix to “Coral Reefs.” The names not printed in
upper case in the Index refer to the Appendix.
THE STRUCTURE AND DISTRIBUTION OF CORAL REEFS.
INTRODUCTION.
The object of this volume is to describe from my own observation and
the works of others, the principal kinds of coral-reefs, more
especially those occurring in the open ocean, and to explain the origin
of their peculiar forms. I do not here treat of the polypifers, which
construct these vast works, except so far as relates to their
distribution, and to the conditions favourable to their vigorous
growth. Without any distinct intention to classify coral-reefs, most
voyagers have spoken of them under the following heads:
“lagoon-islands,” or “atolls,” “barrier” or “encircling reefs,” and
“fringing” or “shore-reefs.” The lagoon-islands have received much the
most attention; and it is not surprising, for every one must be struck
with astonishment, when he first beholds one of these vast rings of
coral-rock, often many leagues in diameter, here and there surmounted
by a low verdant island with dazzling white shores, bathed on the
outside by the foaming breakers of the ocean, and on the inside
surrounding a calm expanse of water, which from reflection, is of a
bright but pale green colour. The naturalist will feel this
astonishment more deeply after having examined the soft and almost
gelatinous bodies of these apparently insignificant creatures, and when
he knows that the solid reef increases only on the outer edge, which
day and night is lashed by the breakers of an ocean never at rest. Well
did François Pyrard de Laval, in the year 1605, exclaim, “C’est une
merueille de voir chacun de ces atollons, enuironné d’un grand banc de
pierre tout autour, n’y ayant point d’artifice humain.” The
accompanying sketch of Whitsunday island, in the South Pacific, taken
from Captain Beechey’s admirable “Voyage,” although excellent of its
kind, gives but a faint idea of the singular aspect of one of these
lagoon-islands.
[Illustration]
Whitsunday Island is of small size, and the whole circle has been
converted into land, which is a comparatively rare circumstance. As the
reef of a lagoon-island generally supports many separate small islands,
the word “island,” applied to the whole, is often the cause of
confusion; hence I have invariably used in this volume the term
“atoll,” which is the name given to these circular groups of
coral-islets by their inhabitants in the Indian Ocean, and is
synonymous with “lagoon-island.”
[Illustration]
Barrier-reefs, when encircling small islands, have been comparatively
little noticed by voyagers; but they well deserve attention. In their
structure they are little less marvellous than atolls, and they give a
singular and most picturesque character to the scenery of the islands
they surround. In the accompanying sketch, taken from the “Voyage of
the ‘Coquille’,” the reef is seen from within, from one of the high
peaks of the island of Bolabola.[1] Here, as in Whitsunday Island, the
whole of that part of the reef which is visible is converted into land.
This is a circumstance of rare occurrence; more usually a snow-white
line of great breakers, with here and there an islet crowned by
cocoa-nut trees, separates the smooth waters of the lagoon-like channel
from the waves of the open sea. The barrier-reefs of Australia and of
New Caledonia, owing to their enormous dimensions, have excited much
attention: in structure and form they resemble those encircling many of
the smaller islands in the Pacific Ocean.
[1] I have taken the liberty of simplifying the foreground, and
leaving out a mountainous island in the far distance.
With respect to fringing, or shore-reefs, there is little in their
structure which needs explanation; and their name expresses their
comparatively small extension. They differ from barrier-reefs in not
lying so far from the shore, and in not having within a broad channel
of deep water. Reefs also occur around submerged banks of sediment and
of worn-down rock; and others are scattered quite irregularly where the
sea is very shallow; these in most respects are allied to those of the
fringing class, but they are of comparatively little interest.
I have given a separate chapter to each of the above classes, and have
described some one reef or island, on which I possessed most
information, as typical; and have afterwards compared it with others of
a like kind. Although this classification is useful from being obvious,
and from including most of the coral-reefs existing in the open sea, it
admits of a more fundamental division into barrier and atoll-formed
reefs on the one hand, where there is a great apparent difficulty with
respect to the foundation on which they must first have grown; and into
fringing-reefs on the other, where, owing to the nature of the slope of
the adjoining land, there is no such difficulty. The two blue tints and
the red colour (replaced by numbers in this edition.) on the map (Plate
III.), represent this main division, as explained in the beginning of
the last chapter. In the Appendix, every existing coral-reef, except
some on the coast of Brazil not included in the map, is briefly
described in geographical order, as far as I possessed information; and
any particular spot may be found by consulting the Index.
Several theories have been advanced to explain the origin of atolls or
lagoon-islands, but scarcely one to account for barrier-reefs. From the
limited depths at which reef-building polypifers can flourish, taken
into consideration with certain other circumstances, we are compelled
to conclude, as it will be seen, that both in atolls and barrier-reefs,
the foundation on which the coral was primarily attached, has subsided;
and that during this downward movement, the reefs have grown upwards.
This conclusion, it will be further seen, explains most satisfactorily
the outline and general form of atolls and barrier-reefs, and likewise
certain peculiarities in their structure. The distribution, also, of
the different kinds of coral-reefs, and their position with relation to
the areas of recent elevation, and to the points subject to volcanic
eruptions, fully accord with this theory of their origin.[2]
[2] A brief account of my views on coral formations, now published in
my Journal of Researches, was read May 31st, 1837, before the
Geological Society, and an abstract has appeared in the Proceedings.
CHAPTER I.
ATOLLS OR LAGOON-ISLANDS.
SECTION I.—KEELING ATOLL.
Corals on the outer margin.—Zone of Nulliporæ.—Exterior
reef.—Islets.—Coral-conglomerate.—Lagoon.—Calcareous sediment.—Scari
and Holuthuriæ subsisting on corals.—Changes in the condition of the
reefs and islets.—Probable subsidence of the atoll.—Future state of the
lagoon.
[Illustration]
A.—Level of the sea at low water: where the letter A is placed, the
depth is twenty-five fathoms, and the distance rather more than one
hundred and fifty yards from the edge of the reef.
B.—Outer edge of that flat part of the reef, which dries at low water:
the edge either consists of a convex mound, as represented, or of
rugged points, like those a little farther seaward, beneath the water.
C.—A flat of coral-rock, covered at high water.
D.—A low projecting ledge of brecciated coral-rock washed by the waves
at high water.
E.—A slope of loose fragments, reached by the sea only during gales:
the upper part, which is from six to twelve feet high, is clothed with
vegetation. The surface of the islet gently slopes to the lagoon.
F.—Level of the lagoon at low water.
Keeling or Cocos atoll is situated in the Indian Ocean, in 12° 5′ S.,
and longitude 90° 55′ E.: a reduced chart of it was made from the
survey of Captain Fitzroy and the Officers of H.M.S. “Beagle,” is given
in Plate I., Figure 10. The greatest width of this atoll is nine miles
and a half. Its structure is in most respects characteristic of the
class to which it belongs, with the exception of the shallowness of the
lagoon. The accompanying woodcut represents a vertical section,
supposed to be drawn at low water from the outer coast across one of
the low islets (one being taken of average dimensions) to within the
lagoon.
The section is true to the scale in a horizontal line, but it could not
be made so in a vertical one, as the average greatest height of the
land is only between six and twelve feet above high-water mark.
I will describe the section, commencing with the outer margin. I must
first observe that the reef-building polypifers, not being tidal
animals, require to be constantly submerged or washed by the breakers.
I was assured by Mr. Liesk, a very intelligent resident on these
islands, as well as by some chiefs at Tahiti (Otaheite), that an
exposure to the rays of the sun for a very short time invariably causes
their destruction. Hence it is possible only under the most favourable
circumstances, afforded by an unusually low tide and smooth water, to
reach the outer margin, where the coral is alive. I succeeded only
twice in gaining this part, and found it almost entirely composed of a
living Porites, which forms great irregularly rounded masses (like
those of an Astræa, but larger) from four to eight feet broad, and
little less in thickness. These mounds are separated from each other by
narrow crooked channels, about six feet deep, most of which intersect
the line of reef at right angles. On the furthest mound, which I was
able to reach by the aid of a leaping-pole, and over which the sea
broke with some violence, although the day was quite calm and the tide
low, the polypifers in the uppermost cells were all dead, but between
three and four inches lower down on its side they were living, and
formed a projecting border round the upper and dead surface. The coral
being thus checked in its upward growth, extends laterally, and hence
most of the masses, especially those a little further inwards, had
broad flat dead summits. On the other hand I could see, during the
recoil of the breakers, that a few yards further seaward, the whole
convex surface of the Porites was alive; so that the point where we
were standing was almost on the exact upward and shoreward limit of
existence of those corals which form the outer margin of the reef. We
shall presently see that there are other organic productions, fitted to
bear a somewhat longer exposure to the air and sun.
Next, but much inferior in importance to the Porites, is the _Millepora
complanata_.[1] It grows in thick vertical plates, intersecting each
other at various angles, and forms an exceedingly strong honeycombed
mass, which generally affects a circular form, the marginal plates
alone being alive. Between these plates and in the protected crevices
on the reef, a multitude of branching zoophytes and other productions
flourish, but the Porites and Millepora alone seem able to resist the
fury of the breakers on its upper and outer edge: at the depth of a few
fathoms other kinds of stony corals live. Mr. Liesk, who was intimately
acquainted with every part of this reef, and likewise with that of
North Keeling atoll, assured me that these corals invariably compose
the outer margin. The lagoon is inhabited by quite a distinct set of
corals, generally brittle and thinly branched; but a Porites,
apparently of the same species with that on the outside, is found
there, although it does not seem to thrive, and certainly does not
attain the thousandth part in bulk of the masses opposed to the
breakers.
[1] This Millepora (Palmipora of Blainville), as well as the _M.
alcicornis_, possesses the singular property of stinging the skin
where it is delicate, as on the face and arm.
The woodcut shows the form of the bottom off the reef: the water
deepens for a space between one and two hundred yards wide, very
gradually to twenty-five fathoms (_A_ in section), beyond which the
sides plunge into the unfathomable ocean at an angle of 45°.[2] To the
depth of ten or twelve fathoms the bottom is exceedingly rugged, and
seems formed of great masses of living coral, similar to those on the
margin. The arming of the lead here invariably came up quite clean, but
deeply indented, and chains and anchors which were lowered, in the
hopes of tearing up the coral, were broken. Many small fragments,
however, of _Millepora alcicornis_ were brought up; and on the arming
from an eight-fathom cast, there was a perfect impression of an Astræa,
apparently alive. I examined the rolled fragments cast on the beach
during gales, in order further to ascertain what corals grew outside
the reef. The fragments consisted of many kinds, of which the Porites
already mentioned and a Madrepora, apparently the _M. corymbosa_, were
the most abundant. As I searched in vain in the hollows on the reef and
in the lagoon, for a living specimen of this Madrepore, I conclude that
it is confined to a zone outside, and beneath the surface, where it
must be very abundant. Fragments of the _Millepora alcicornis_ and of
an Astræa were also numerous; the former is found, but not in
proportionate numbers, in the hollows on the reef; but the Astræa I did
not see living. Hence we may infer, that these are the kinds of coral
which form the rugged sloping surface (represented in the woodcut by an
uneven line), round and beneath the external margin. Between twelve and
twenty fathoms the arming came up an equal number of times smoothed
with sand, and indented with coral: an anchor and lead were lost at the
respective depths of thirteen and sixteen fathoms. Out of twenty-five
soundings taken at a greater depth than twenty fathoms, every one
showed that the bottom was covered with sand; whereas, at a less depth
than twelve fathoms, every sounding showed that it was exceedingly
rugged, and free from all extraneous particles. Two soundings were
obtained at the depth of 360 fathoms, and several between two hundred
and three hundred fathoms. The sand brought up from these depths
consisted of finely triturated fragments of stony zoophytes, but not,
as far as I could distinguish, of a particle of any lamelliform genus:
fragments of shells were rare.
[2] The soundings from which this section is laid down were taken with
great care by Captain Fitzroy himself. He used a bell-shaped lead,
having a diameter of four inches, and the armings each time were cut
off and brought on board for me to examine. The arming is a
preparation of tallow, placed in the concavity at the bottom of the
lead. Sand, and even small fragments of rock, will adhere to it; and
if the bottom be of rock it brings up an exact impression of its
surface.
At a distance of 2,200 yards from the breakers, Captain Fitzroy found
no bottom with a line of 7,200 feet in length; hence the submarine
slope of this coral formation is steeper than that of any volcanic
cone. Off the mouth of the lagoon, and likewise off the northern point
of the atoll, where the currents act violently, the inclination, owing
to the accumulation of sediment, is less. As the arming of the lead
from all the greater depths showed a smooth sandy bottom, I at first
concluded that the whole consisted of a vast conical pile of calcareous
sand, but the sudden increase of depth at some points, and the
circumstance of the line having been cut, as if rubbed, when between
five hundred and six hundred fathoms were out, indicate the probable
existence of submarine cliffs.
On the margin of the reef, close within the line where the upper
surface of the Porites and of the Millepora is dead, three species of
Nullipora flourish. One grows in thin sheets, like a lichen on old
trees; the second in stony knobs, as thick as a man’s finger, radiating
from a common centre; and the third, which is less common, in a
moss-like reticulation of thin, but perfectly rigid branches.[3] The
three species occur either separately or mingled together; and they
form by their successive growth a layer two or three feet in thickness,
which in some cases is hard, but where formed of the lichen-like kind,
readily yields an impression to the hammer: the surface is of a reddish
colour. These Nulliporæ, although able to exist above the limit of true
corals, seem to require to be bathed during the greater part of each
tide by breaking water, for they are not found in any abundance in the
protected hollows on the back part of the reef, where they might be
immersed either during the whole or an equal proportional time of each
tide. It is remarkable that organic productions of such extreme
simplicity, for the Nulliporæ undoubtedly belong to one of the lowest
classes of the vegetable kingdom, should be limited to a zone so
peculiarly circumstanced. Hence the layer composed by their growth
merely fringes the reef for a space of about twenty yards in width,
either under the form of separate mammillated projections, where the
outer masses of coral are separate, or, more commonly, where the corals
are united into a solid margin, as a continuous smooth convex mound
(_B_ in woodcut), like an artificial breakwater. Both the mound and
mammillated projections stand about three feet higher than any other
part of the reef, by which term I do not include the islets, formed by
the accumulation of rolled fragments. We shall hereafter see that other
coral reefs are protected by a similar thick growth of Nulliporæ on the
outer margin, the part most exposed to the breakers, and this must
effectually aid in preserving it from being worn down.
[3] This last species is of a beautiful bright peach-blossom colour.
Its branches are about as thick as crow-quills; they are slightly
flattened and knobbed at the extremities. The extremities only are
alive and brightly coloured. The two other species are of a dirty
purplish-white. The second species is extremely hard; its short
knob-like branches are cylindrical, and do not grow thicker at their
extremities.
The woodcut represents a section across one of the islets on the reef,
but if all that part which is above the level of _C_ were removed, the
section would be that of a simple reef, as it occurs where no islet has
been formed. It is this reef which essentially forms the atoll. It is a
ring, enclosing the lagoon on all sides except at the northern end,
where there are two open spaces, through one of which ships can enter.
The reef varies in width from two hundred and fifty to five hundred
yards, its surface is level, or very slightly inclined towards the
lagoon, and at high tide the sea breaks entirely over it: the water at
low tide thrown by the breakers on the reef, is carried by the many
narrow and shoal gullies or channels on its surface, into the lagoon: a
return stream sets out of the lagoon through the main entrance. The
most frequent coral in the hollows on the reef is _Pocillopora
verrucosa_, which grows in short sinuous plates, or branches, and when
alive is of a beautiful pale lake-red: a Madrepora, closely allied or
identical with _M. pocillifera_, is also common. As soon as an islet is
formed, and the waves are prevented breaking entirely over the reef,
the channels and hollows in it become filled up with cemented
fragments, and its surface is converted into a hard smooth floor (_C_
of woodcut), like an artificial one of freestone. This flat surface
varies in width from one hundred to two hundred, or even three hundred
yards, and is strewed with a few large fragments of coral torn up
during gales: it is uncovered only at low water. I could with
difficulty, and only by the aid of a chisel, procure chips of rock from
its surface, and therefore could not ascertain how much of it is formed
by the aggregation of detritus, and how much by the outward growth of
mounds of corals, similar to those now living on the margin. Nothing
can be more singular than the appearance at low tide of this “flat” of
naked stone, especially where it is externally bounded by the smooth
convex mound of Nulliporæ, appearing like a breakwater built to resist
the waves, which are constantly throwing over it sheets of foaming
water. The characteristic appearance of this “flat” is shown in the
foregoing woodcut of Whitsunday atoll.
The islets on the reef are first formed between two hundred and three
hundred yards from its outer edge, through the accumulation of a pile
of fragments, thrown together by some unusually strong gale. Their
ordinary width is under a quarter of a mile, and their length varies
from a few yards to several miles. Those on the south-east and windward
side of the atoll, increase solely by the addition of fragments on
their outer side; hence the loose blocks of coral, of which their
surface is composed, as well as the shells mingled with them, almost
exclusively consist of those kinds which live on the outer coast. The
highest part of the islets (excepting hillocks of blown sand, some of
which are thirty feet high), is close to the outer beach (_E_ of the
woodcut), and averages from six to ten feet above ordinary high-water
mark. From the outer beach the surface slopes gently to the shores of
the lagoon, which no doubt has been caused by the breakers the further
they have rolled over the reef, having had less power to throw up
fragments. The little waves of the lagoon heap up sand and fragments of
thinly-branched corals on the inner side of the islets on the leeward
side of the atoll; and these islets are broader than those to windward,
some being even eight hundred yards in width; but the land thus added
is very low. The fragments beneath the surface are cemented into a
solid mass, which is exposed as a ledge (_D_ of the woodcut),
projecting some yards in front of the outer shore and from two to four
feet high. This ledge is just reached by the waves at ordinary
high-water: it extends in front of all the islets, and everywhere has a
water-worn and scooped appearance. The fragments of coral which are
occasionally cast on the “flat” are during gales of unusual violence
swept together on the beach, where the waves each day at high-water
tend to remove and gradually wear them down; but the lower fragments
having become firmly cemented together by the percolation of calcareous
matter, resist the daily tides longer, and hence project as a ledge.
The cemented mass is generally of a white colour, but in some few parts
reddish from ferruginous matter; it is very hard, and is sonorous under
the hammer; it is obscurely divided by seams, dipping at a small angle
seaward; it consists of fragments of the corals which grow on the outer
margin, some quite and others partially rounded, some small and others
between two and three feet across; and of masses of previously formed
conglomerate, torn up, rounded, and re-cemented; or it consists of a
calcareous sandstone, entirely composed of rounded particles, generally
almost blended together, of shells, corals, the spines of echini, and
other such organic bodies; rocks, of this latter kind, occur on many
shores, where there are no coral reefs. The structure of the coral in
the conglomerate has generally been much obscured by the infiltration
of spathose calcareous matter; and I collected a very interesting
series, beginning with fragments of unaltered coral, and ending with
others, where it was impossible to discover with the naked eye any
trace of organic structure. In some specimens I was unable, even with
the aid of a lens, and by wetting them, to distinguish the boundaries
of the altered coral and spathose limestone. Many even of the blocks of
coral lying loose on the beach, had their central parts altered and
infiltrated.
The lagoon alone remains to be described; it is much shallower than
that of most atolls of considerable size. The southern part is almost
filled up with banks of mud and fields of coral, both dead and alive,
but there are considerable spaces, between three and four fathoms, and
smaller basins, from eight to ten fathoms deep. Probably about half its
area consists of sediment, and half of coral-reefs. The corals
composing these reefs have a very different aspect from those on the
outside; they are very numerous in kind, and most of them are thinly
branched. Meandrina, however, lives in the lagoon, and great rounded
masses of this coral are numerous, lying quite or almost loose on the
bottom. The other commonest kinds consist of three closely allied
species of true Madrepora in thin branches; of _Seriatapora subulata;_
two species of Porites[4] with cylindrical branches, one of which forms
circular clumps, with the exterior branches only alive; and lastly, a
coral something like an Explanaria, but with stars on both surfaces,
growing in thin, brittle, stony, foliaceous expansions, especially in
the deeper basins of the lagoon. The reefs on which these corals grow
are very irregular in form, are full of cavities, and have not a solid
flat surface of dead rock, like that surrounding the lagoon; nor can
they be nearly so hard, for the inhabitants made with crowbars a
channel of considerable length through these reefs, in which a
schooner, built on the S.E. islet, was floated out. It is a very
interesting circumstance, pointed out to us by Mr. Liesk, that this
channel, although made less than ten years before our visit, was then,
as we saw, almost choked up with living coral, so that fresh
excavations would be absolutely necessary to allow another vessel to
pass through it.
[4] This Porites has somewhat the habit of _P. clavaria_, but the
branches are not knobbed at their ends. When alive it is of a yellow
colour, but after having been washed in fresh water and placed to dry,
a jet-black slimy substance exuded from the entire surface, so that
the specimen now appears as if it had been dipped in ink.
The sediment from the deepest parts in the lagoon, when wet, appeared
chalky, but when dry, like very fine sand. Large soft banks of similar,
but even finer grained mud, occur on the S.E. shore of the lagoon,
affording a thick growth of a Fucus, on which turtle feed: this mud,
although discoloured by vegetable matter, appears from its entire
solution in acids to be purely calcareous. I have seen in the Museum of
the Geological Society, a similar but more remarkable substance,
brought by Lieutenant Nelson from the reefs of Bermuda, which, when
shown to several experienced geologists, was mistaken by them for true
chalk. On the outside of the reef much sediment must be formed by the
action of the surf on the rolled fragments of coral; but in the calm
waters of the lagoon, this can take place only in a small degree. There
are, however, other and unexpected agents at work here: large shoals of
two species of Scarus, one inhabiting the surf outside the reef and the
other the lagoon, subsist entirely, as I was assured by Mr. Liesk, the
intelligent resident before referred to, by browsing on the living
polypifers. I opened several of these fish, which are very numerous and
of considerable size, and I found their intestines distended by small
pieces of coral, and finely ground calcareous matter. This must daily
pass from them as the finest sediment; much also must be produced by
the infinitely numerous vermiform and molluscous animals, which make
cavities in almost every block of coral. Dr. J. Allan, of Forres, who
has enjoyed the best means of observation, informs me in a letter that
the Holothuriæ (a family of Radiata) subsist on living coral; and the
singular structure of bone within the anterior extremity of their
bodies, certainly appears well adapted for this purpose. The number of
the species of Holothuria, and of the individuals which swarm on every
part of these coral-reefs, is extraordinarily great; and many shiploads
are annually freighted, as is well-known, for China with the trepang,
which is a species of this genus. The amount of coral yearly consumed,
and ground down into the finest mud, by these several creatures, and
probably by many other kinds, must be immense. These facts are,
however, of more importance in another point of view, as showing us
that there are living checks to the growth of coral-reefs, and that the
almost universal law of “consumed and be consumed,” holds good even
with the polypifers forming those massive bulwarks, which are able to
withstand the force of the open ocean.
Considering that Keeling atoll, like other coral formations, has been
entirely formed by the growth of organic beings, and the accumulation
of their detritus, one is naturally led to inquire how long it has
continued, and how long it is likely to continue, in its present state.
Mr. Liesk informed me that he had seen an old chart in which the
present long island on the S.E. side was divided by several channels
into as many islets; and he assures me that the channels can still be
distinguished by the smaller size of the trees on them. On several
islets, also, I observed that only young cocoa-nut trees were growing
on the extremities; and that older and taller trees rose in regular
succession behind them; which shows that these islets have very lately
increased in length. In the upper and south-eastern part of the lagoon,
I was much surprised by finding an irregular field of at least a mile
square of branching corals, still upright, but entirely dead. They
consisted of the species already mentioned; they were of a brown
colour, and so rotten, that in trying to stand on them I sank halfway
up the leg, as if through decayed brushwood. The tops of the branches
were barely covered by water at the time of lowest tide. Several facts
having led me to disbelieve in any elevation of the whole atoll, I was
at first unable to imagine what cause could have killed so large a
field of coral. Upon reflection, however, it appeared to me that the
closing up of the above-mentioned channels would be a sufficient cause;
for before this, a strong breeze by forcing water through them into the
head of the lagoon, would tend to raise its level. But now this cannot
happen, and the inhabitants observe that the tide rises to a less
height, during a high S.E. wind, at the head than at the mouth of the
lagoon. The corals, which, under the former condition of things, had
attained the utmost possible limit of upward growth, would thus
occasionally be exposed for a short time to the sun, and be killed.
Besides the increase of dry land, indicated by the foregoing facts, the
exterior solid reef appears to have grown outwards. On the western side
of the atoll, the “flat” lying between the margin of the reef and the
beach, is very wide; and in front of the regular beach with its
conglomerate basis, there is, in most parts, a bed of sand and loose
fragments with trees growing out of it, which apparently is not reached
even by the spray at high water. It is evident some change has taken
place since the waves formed the inner beach; that they formerly beat
against it with violence was evident, from a remarkably thick and
water-worn point of conglomerate at one spot, now protected by
vegetation and a bank of sand; that they beat against it in the same
peculiar manner in which the swell from windward now obliquely curls
round the margin of the reef, was evident from the conglomerate having
been worn into a point projecting from the beach in a similarly oblique
manner. This retreat in the line of action of the breakers might
result, either from the surface of the reef in front of the islets
having been submerged at one time, and afterward having grown upwards,
or from the mounds of coral on the margin having continued to grow
outwards. That an outward growth of this part is in process, can hardly
be doubted from the fact already mentioned of the mounds of Porites
with their summits apparently lately killed, and their sides only three
or four inches lower down thickened by a fresh layer of living coral.
But there is a difficulty on this supposition which I must not pass
over. If the whole, or a large part of the “flat,” had been formed by
the outward growth of the margin, each successive margin would
naturally have been coated by the Nulliporæ, and so much of the surface
would have been of equal height with the existing zone of living
Nulliporæ: this is not the case, as may be seen in the woodcut. It is,
however, evident from the abraded state of the “flat,” with its
original inequalities filled up, that its surface has been much
modified; and it is possible that the hinder portions of the zone of
Nulliporæ, perishing as the reef grows outwards, might be worn down by
the surf. If this has not taken place, the reef can in no part have
increased outwards in breadth since its formation, or at least since
the Nulliporæ formed the convex mound on its margin; for the zone thus
formed, and which stands between two and three feet above the other
parts of the reef, is nowhere much above twenty yards in width.
Thus far we have considered facts, which indicate, with more or less
probability, the increase of the atoll in its different parts: there
are others having an opposite tendency. On the south-east side,
Lieutenant Sulivan, to whose kindness I am indebted for many
interesting observations, found the conglomerate projecting on the reef
nearly fifty yards in front of the beach: we may infer from what we see
in all other parts of the atoll, that the conglomerate was not
originally so much exposed, but formed the base of an islet, the front
and upper part of which has since been swept away. The degree to which
the conglomerate, round nearly the whole atoll, has been scooped,
broken up, and the fragments cast on the beach, is certainly very
surprising, even on the view that it is the office of occasional gales
to pile up fragments, and of the daily tides to wear them away. On the
western side, also, of the atoll, where I have described a bed of sand
and fragments with trees growing out of it, in front of an old beach,
it struck both Lieutenant Sulivan and myself, from the manner in which
the trees were being washed down, that the surf had lately recommenced
an attack on this line of coast. Appearances indicating a slight
encroachment of the water on the land, are plainer within the lagoon: I
noticed in several places, both on its windward and leeward shores, old
cocoa-nut trees falling with their roots undermined, and the rotten
stumps of others on the beach, where the inhabitants assured us the
cocoa-nut could not now grow. Captain Fitzroy pointed out to me, near
the settlement, the foundation posts of a shed, now washed by every
tide, but which the inhabitants stated, had seven years before stood
above high watermark. In the calm waters of the lagoon, directly
connected with a great, and therefore stable ocean, it seems very
improbable that a change in the currents, sufficiently great to cause
the water to eat into the land on all sides, should have taken place
within a limited period. From these considerations I inferred, that
probably the atoll had lately subsided to a small amount; and this
inference was strengthened by the circumstance, that in 1834, two years
before our visit, the island had been shaken by a severe earthquake,
and by two slighter ones during the ten previous years. If, during
these subterranean disturbances, the atoll did subside, the downward
movement must have been very small, as we must conclude from the fields
of dead coral still lipping the surface of the lagoon, and from the
breakers on the western shore not having yet regained the line of their
former action. The subsidence must, also, have been preceded by a long
period of rest, during which the islets extended to their present size,
and the living margin of the reef grew either upwards, or as I believe
outwards, to its present distance from the beach.
Whether this view be correct or not, the above facts are worthy of
attention, as showing how severe a struggle is in progress on these low
coral formations between the two nicely balanced powers of land and
water. With respect to the future state of Keeling atoll, if left
undisturbed, we can see that the islets may still extend in length; but
as they cannot resist the surf until broken by rolling over a wide
space, their increase in breadth must depend on the increasing breadth
of the reef; and this must be limited by the steepness of the submarine
flanks, which can be added to only by sediment derived from the wear
and tear of the coral. From the rapid growth of the coral in the
channel cut for the schooner, and from the several agents at work in
producing fine sediment, it might be thought that the lagoon would
necessarily become quickly filled up. Some of this sediment, however,
is transported into the open sea, as appears from the soundings off the
mouth of the lagoon, instead of being deposited within it. The
deposition, moreover, of sediment, checks the growth of coral-reefs, so
that these two agencies cannot act together with full effect in filling
it up. We know so little of the habits of the many different species of
corals, which form the lagoon-reefs, that we have no more reasons for
supposing that their whole surface would grow up as quickly as the
coral did in the schooner-channel, than for supposing that the whole
surface of a peat-moss would increase as quickly as parts are known to
do in holes, where the peat has been cut away. These agencies,
nevertheless, tend to fill up the lagoon; but in proportion as it
becomes shallower, so must the polypifers be subject to many injurious
agencies, such as impure water and loss of food. For instance, Mr.
Liesk informed me, that some years before our visit unusually heavy
rain killed nearly all the fish in the lagoon, and probably the same
cause would likewise injure the corals. The reefs also, it must be
remembered, cannot possibly rise above the level of the lowest
spring-tide, so that the final conversion of the lagoon into land must
be due to the accumulation of sediment; and in the midst of the clear
water of the ocean, and with no surrounding high land, this process
must be exceedingly slow.
SECTION II.—GENERAL DESCRIPTION OF ATOLLS.
General form and size of atolls, their reefs and islets.—External
slope.— Zone of Nulliporæ.—Conglomerate.—Depth of
lagoons.—Sediment.—Reefs submerged wholly or in part.—Breaches in the
reef.—Ledge-formed shores round certain lagoons.—Conversion of lagoons
into land.
I will here give a sketch of the general form and structure of the many
atolls and atoll-formed reefs which occur in the Pacific and Indian
Oceans, comparing them with Keeling atoll. The Maldiva atolls and the
Great Chagos Bank differ in so many respects, that I shall devote to
them, besides occasional references, a third section of this chapter.
Keeling atoll may be considered as of moderate dimensions and of
regular form. Of the thirty-two islands surveyed by Captain Beechey in
the Low Archipelago, the longest was found to be thirty miles, and the
shortest less than a mile; but Vliegen atoll, situated in another part
of the same group, appears to be sixty miles long and twenty broad.
Most of the atolls in this group are of an elongated form; thus Bow
Island is thirty miles in length, and on an average only six in width
(See Figure 4, Plate I.), and Clermont Tonnere has nearly the same
proportions. In the Marshall Archipelago (the Ralick and Radack group
of Kotzebue) several of the atolls are more than thirty miles in
length, and Rimsky Korsacoff is fifty-four long, and twenty wide, at
the broadest part of its irregular outline. Most of the atolls in the
Maldiva Archipelago are of great size, one of them (which, however,
bears a double name) measured in a medial and slightly curved line, is
no less than eighty-eight geographical miles long, its greatest width
being under twenty, and its least only nine and a half miles. Some
atolls have spurs projecting from them; and in the Marshall group there
are atolls united together by linear reefs, for instance Menchikoff
Island (See Figure 3, Plate II.), which is sixty miles in length, and
consists of three loops tied together. In far the greater number of
cases an atoll consists of a simple elongated ring, with its outline
moderately regular.
The average width of the annular wreath may be taken as about a quarter
of a mile. Captain Beechey[5] says that in the atolls of the Low
Archipelago it exceeded in no instance half a mile. The description
given of the structure and proportional dimensions of the reef and
islets of Keeling atoll, appears to apply perfectly to nearly all the
atolls in the Pacific and Indian Oceans. The islets are first formed
some way back either on the projecting points of the reef, especially
if its form be angular, or on the sides of the main entrances into the
lagoon—that is in both cases, on points where the breakers can act
during gales of wind in somewhat different directions, so that the
matter thrown up from one side may accumulate against that before
thrown up from another. In Lutké’s chart of the Caroline atolls, we see
many instances of the former case; and the occurrence of islets, as if
placed for beacons, on the points where there is a gateway or breach
through the reef, has been noticed by several authors. There are some
atoll-formed reefs, rising to the surface of the sea and partly dry at
low water, on which from some cause islets have never been formed; and
there are others on which they have been formed, but have subsequently
been worn away. In atolls of small dimensions the islets frequently
become united into a single horse-shoe or ring-formed strip; but Diego
Garcia, although an atoll of considerable size, being thirteen miles
and a half in length, has its lagoon entirely surrounded, except at the
northern end, by a belt of land, on an average a third of a mile in
width. To show how small the total area of the annular reef and the
land is in islands of this class, I may quote a remark from the voyage
of Lutké, namely, that if the forty-three rings, or atolls, in the
Caroline Archipelago, were put one within another, and over a steeple
in the centre of St. Petersburg, the whole world would not cover that
city and its suburbs.
[5] Beechey’s “Voyage to the Pacific and Beering’s Straits,” chapter
viii.
The form of the bottom off Keeling atoll, which gradually slopes to
about twenty fathoms at the distance of between one and two hundred
yards from the edge of the reef, and then plunges at an angle of 45°
into unfathomable depths, is exactly the same[6] with that of the
sections of the atolls in the Low Archipelago given by Captain Beechey.
The nature, however, of the bottom seems to differ, for this officer[7]
informs me that all the soundings, even the deepest, were on coral, but
he does not know whether dead or alive. The slope round Christmas atoll
(Lat. 1° 4′ N., 157° 45′ W.), described by Cook,[8] is considerably
less, at about half a mile from the edge of the reef, the average depth
was about fourteen fathoms on a fine sandy bottom, and at a mile, only
between twenty and forty fathoms. It has no doubt been owing to this
gentle slope, that the strip of land surrounding its lagoon, has
increased in one part to the extraordinary width of three miles; it is
formed of successive ridges of broken shells and corals, like those on
the beach. I know of no other instance of such width in the reef of an
atoll; but Mr. F.D. Bennett informs me that the inclination of the
bottom round Caroline atoll in the Pacific, is like that off Christmas
Island, very gentle. Off the Maldiva and Chagos atolls, the inclination
is much more abrupt; thus at Heawandoo Pholo, Lieutenant Powell[9]
found fifty and sixty fathoms close to the edge of the reef, and at 300
yards distance there was no bottom with a 300-yard line. Captain
Moresby informs me, that at 100 fathoms from the mouth of the lagoon of
Diego Garcia, he found no bottom with 150 fathoms; this is the more
remarkable, as the slope is generally less abrupt in front of channels
through a reef, owing to the accumulation of sediment. At Egmont
Island, also, at 150 fathoms from the reef, soundings were struck with
150 fathoms. Lastly, at Cardoo atoll, only sixty yards from the reef,
no bottom was obtained, as I am informed by Captain Moresby, with a
line of 200 fathoms! The currents run with great force round these
atolls, and where they are strongest, the inclination appears to be
most abrupt. I am informed by the same authority, that wherever
soundings were obtained off these islands, the bottom was invariably
sandy: nor was there any reason to suspect the existence of submarine
cliffs, as there was at Keeling Island.[10] Here then occurs a
difficulty; can sand accumulate on a slope, which, in some cases,
appears to exceed fifty-five degrees? It must be observed, that I speak
of slopes where soundings were obtained, and not of such cases, as that
of Cardoo, where the nature of the bottom is unknown, and where its
inclination must be nearly vertical. M. Élie de Beaumont[11] has
argued, and there is no higher authority on this subject, from the
inclination at which snow slides down in avalanches, that a bed of sand
or mud cannot be formed at a greater angle than thirty degrees.
Considering the number of soundings on sand, obtained round the Maldiva
and Chagos atolls, which appears to indicate a greater angle, and the
extreme abruptness of the sand-banks in the West Indies, as will be
mentioned in the Appendix, I must conclude that the adhesive property
of wet sand counteracts its gravity, in a much greater ratio than has
been allowed for by M. Élie de Beaumont. From the facility with which
calcareous sand becomes agglutinated, it is not necessary to suppose
that the bed of loose sand is thick.
[6] The form of the bottom round the Marshall atolls in the Northern
Pacific is probably similar: Kotzebue (“First Voyage,” volume ii.,
page 16.) says: “We had at a small distance from the reef, forty
fathoms depth, which increased a little further so much that we could
find no bottom.”)
[7] I must be permitted to express my obligation to Captain Beechey,
for the very kind manner in which he has given me information on
several points, and to own the great assistance I have derived from
his excellent published work.
[8] Cook’s “Third Voyage,” volume ii., chapter 10.
[9] This fact is taken from a MS. account of these groups lent me by
Captain Moresby. See also Captain Moresby’s paper on the Maldiva
atolls in the “Geographical Journal”, volume v., page 401.
[10] Off some of the islands in the Low Archipelago the bottom appears
to descend by ledges. Off Elizabeth Island, which, however, consists
of raised coral, Captain Beechey (p. 45, quarto edition) describes
three ledges: the first had an easy slope from the beach to a distance
of about fifty yards: the second extended two hundred yards with
twenty-five fathoms on it, and then ended abruptly, like the first;
and immediately beyond this there was no bottom with two hundred
fathoms.
[11] “Mémoires pour servir à une description Géolog. de France,” tome
iv., page 216.
Captain Beechey has observed, that the submarine slope is much less at
the extremities of the more elongated atolls in the Low Archipelago,
than at their sides; in speaking of Ducie’s Island he says[12] the
buttress, as it may be called, which “has the most powerful enemy (the
S.W. swell) to oppose, is carried out much further, and with less
abruptness than the other.” In some cases, the less inclination of a
certain part of the external slope, for instance of the northern
extremities of the two Keeling atolls, is caused by a prevailing
current which there accumulates a bed of sand. Where the water is
perfectly tranquil, as within a lagoon, the reefs generally grow up
perpendicularly, and sometimes even overhang their bases; on the other
hand, on the leeward side of Mauritius, where the water is generally
tranquil, although not invariably so, the reef is very gently inclined.
Hence it appears that the exterior angle varies much; nevertheless in
the close similarity in form between the sections of Keeling atoll and
of the atolls in the Low Archipelago, in the general steepness of the
reefs of the Maldiva and Chagos atolls, and in the perpendicularity of
those rising out of water always tranquil, we may discern the effects
of uniform laws; but from the complex action of the surf and currents,
on the growing powers of the coral and on the deposition of sediment,
we can by no means follow out all the results.
[12] Beechey’s “Voyage,” 4to edition, page 44.
Where islets have been formed on the reef, that part which I have
sometimes called the “flat” and which is partly dry at low water,
appears similar in every atoll. In the Marshall group in the North
Pacific, it may be inferred from Chamisso’s description, that the reef,
where islets have not been formed on it, slopes gently from the
external margin to the shores of the lagoon; Flinders states that the
Australian barrier has a similar inclination inwards, and I have no
doubt it is of general occurrence, although, according to Ehrenberg,
the reefs of the Red Sea offer an exception. Chamisso observes that
“the red colour of the reef (at the Marshall atolls) under the breakers
is caused by a Nullipora, which covers the stone _wherever the waves
beat;_ and, under favourable circumstances, assumes a stalactical
form,”—a description perfectly applicable to the margin of Keeling
atoll.[13] Although Chamisso does not state that the masses of
Nulliporæ form points or a mound, higher than the flat, yet I believe
that this is the case; for Kotzebue,[14] in another part, speaks of the
rocks on the edge of the reef “as visible for about two feet at low
water,” and these rocks we may feel quite certain are not formed of
true coral.[15] Whether a smooth convex mound of Nulliporæ, like that
which appears as if artificially constructed to protect the margin of
Keeling Island, is of frequent occurrence round atolls, I know not; but
we shall presently meet with it, under precisely the same form, on the
outer edge of the “barrier-reefs” which encircle the Society Islands.
[13] Kotzebue’s “First Voyage,” volume iii., page 142. Near Porto
Praya, in the Cape de Verde Islands, some basaltic rocks, lashed by no
inconsiderable surf, were completely enveloped with a layer of
Nulliporæ. The entire surface over many square inches, was coloured of
a peach-blossomed red; the layer, however, was of no greater thickness
than paper. Another kind, in the form of projecting knobs, grew in the
same situation. These Nulliporæ are closely related to those described
on the coral-reefs, but I believe are of different species.
[14] Kotzebue, “First Voyage,” volume ii., page 16. Lieutenant Nelson,
in his excellent memoir in the Geological Transactions (volume ii.,
page 105), alludes to the rocky points mentioned by Kotzebue, and
infers that they consist of Serpulæ, which compose incrusting masses
on the reefs of Bermudas, as they likewise do on a sandstone bar off
the coast of Brazil (which I have described in “London Phil. Journal,”
October 1841). These masses of Serpulæ hold the same position,
relatively to the action of the sea, with the Nulliporæ on the
coral-reefs in the Indian and Pacific Oceans.
[15] Captain Moresby, in his valuable paper “on the Northern atolls of
Maldivas” (“Geographical Journal”, volume v.), says that the edges of
the reefs there stand above water at low spring-tides.
There appears to be scarcely a feature in the structure of Keeling
reef, which is not of common, if not of universal occurrence, in other
atolls. Thus Chamisso describes[16] a layer of coarse conglomerate,
outside the islets round the Marshall atolls which “appears on its
upper surface uneven and eaten away.” From drawings, with appended
remarks, of Diego Garcia in the Chagos group and of several of the
Maldiva atolls, shown me by Captain Moresby,[17] it is evident that
their outer coasts are subject to the same round of decay and
renovation as those of Keeling atoll. From the description of the
atolls in the Low Archipelago, given in Captain Beechey’s “Voyage,” it
is not apparent that any conglomerate coral-rock was there observed.
[16] Kotzebue’s “First Voyage,” volume iii., page 144.
[17] See also Moresby on the Northern atolls of the Maldivas,
“Geographical Journal”, volume v., page 400.
The lagoon in Keeling atoll is shallow; in the atolls of the Low
Archipelago the depth varies from 20 to 38 fathoms, and in the Marshall
Group, according to Chamisso, from 30 to 35; in the Caroline atolls it
is only a little less. Within the Maldiva atolls there are large spaces
with 45 fathoms, and some soundings are laid down of 49 fathoms. The
greater part of the bottom in most lagoons, is formed of sediment;
large spaces have exactly the same depth, or the depth varies so
insensibly, that it is evident that no other means, excepting aqueous
deposition, could have leveled the surface so equally. In the Maldiva
atolls this is very conspicuous, and likewise in some of the Caroline
and Marshall Islands. In the former large spaces consist of sand and
_soft clay;_ and Kotzebue speaks of clay having been found within one
of the Marshall atolls. No doubt this clay is calcareous mud, similar
to that at Keeling Island, and to that at Bermuda already referred to,
as undistinguishable from disintegrated chalk, and which Lieutenant
Nelson says is called there pipe-clay.[18]
[18] I may here observe that on the coast of Brazil, where there is
much coral, the soundings near the land are described by Admiral
Roussin, in the _Pilote du Brésil_, as siliceous sand, mingled with
much finely comminuted particles of shells and coral. Further in the
offing, for a space of 1,300 miles along the coast, from the Abrolhos
Islands to Maranham, the bottom in many places is composed of “tuf
blanc, mêlé ou formé de madrépores broyés.” This white substance,
probably, is analogous to that which occurs within the above-mentioned
lagoons; it is sometimes, according to Roussin, firm, and he compares
it to mortar.
Where the waves act with unequal force on the two sides of an atoll,
the islets appear to be first formed, and are generally of greater
continuity on the more exposed shore. The islets, also, which are
placed to leeward, are in most parts of the Pacific liable to be
occasionally swept entirely away by gales, equalling hurricanes in
violence, which blow in an opposite direction to the ordinary
trade-wind. The absence of the islets on the leeward side of atolls, or
when present their lesser dimensions compared with those to windward,
is a comparatively unimportant fact; but in several instances the reef
itself on the leeward side, retaining its usual defined outline, does
not rise to the surface by several fathoms. This is the case with the
southern side of Peros Banhos (Plate I., Figure 9) in the Chagos group,
with Mourileu atoll[19] in the Caroline Archipelago, and with the
barrier-reef (Plate I., Figure 8) of the Gambier Islands. I allude to
the latter reef, although belonging to another class, because Captain
Beechey was first led by it to observe the peculiarity in the question.
At Peros Banhos the submerged part is nine miles in length, and lies at
an average depth of about five fathoms; its surface is nearly level,
and consists of hard stone, with a thin covering of loose sand. There
is scarcely any living coral on it, even on the outer margin, as I have
been particularly assured by Captain Moresby; it is, in fact, a wall of
dead coral-rock, having the same width and transverse section with the
reef in its ordinary state, of which it is a continuous portion. The
living and perfect parts terminate abruptly, and abut on the submerged
portions, in the same manner as on the sides of an ordinary passage
through the reef. The reef to leeward in other cases is nearly or quite
obliterated, and one side of the lagoon is left open; for instance, at
Oulleay (Caroline Archipelago), where a crescent-formed reef is fronted
by an irregular bank, on which the other half of the annular reef
probably once stood. At Namonouïto, in the same Archipelago, both these
modifications of the reef concur; it consists of a great flat bank,
with from twenty to twenty-five fathoms water on it; for a length of
more than forty miles on its southern side it is open and without any
reef, whilst on the other sides it is bounded by a reef, in parts
rising to the surface and perfectly characterised, in parts lying some
fathoms submerged. In the Chagos group there are annular reefs,
entirely submerged, which have the same structure as the submerged and
defined portions just described. The Speaker’s Bank offers an excellent
example of this structure; its central expanse, which is about
twenty-two fathoms deep, is twenty-four miles across; the external rim
is of the usual width of annular reefs, and is well-defined; it lies
between six and eight fathoms beneath the surface, and at the same
depth there are scattered knolls in the lagoon. Captain Moresby
believes the rim consists of dead rock, thinly covered with sand, and
he is certain this is the case with the external rim of the Great
Chagos Bank, which is also essentially a submerged atoll. In both these
cases, as in the submerged portion of the reef at Peros Banhos, Captain
Moresby feels sure that the quantity of living coral, even on the outer
edge overhanging the deep-sea water, is quite insignificant. Lastly, in
several parts of the Pacific and Indian Oceans there are banks, lying
at greater depths than in the cases just mentioned, of the same form
and size with the neighbouring atolls, but with their atoll-like
structure wholly obliterated. It appears from the survey of Freycinet,
that there are banks of this kind in the Caroline Archipelago, and, as
is reported, in the Low Archipelago. When we discuss the origin of the
different classes of coral formations, we shall see that the submerged
state of the whole of some atoll-formed reefs, and of portions of
others, generally but not invariably on the leeward side, and the
existence of more deeply submerged banks now possessing little or no
signs of their original atoll-like structure, are probably the effects
of a uniform cause,—namely, the death of the coral, during the
subsidence of the area, in which the atolls or banks are situated.
[19] Frederick Lutké’s “Voyage autour du Monde,” volume ii., page 291.
See also his account of Namonouïto, below, and the chart of Oulleay in
the Atlas.
There is seldom, with the exception of the Maldiva atolls, more than
two or three channels, and generally only one leading into the lagoon,
of sufficient depth for a ship to enter. in small atolls, there is
usually not even one. Where there is deep water, for instance above
twenty fathoms, in the middle of the lagoon, the channels through the
reef are seldom as deep as the centre,—it may be said that the rim only
of the saucer-shaped hollow forming the lagoon is notched. Mr.
Lyell[20] has observed that the growth of the coral would tend to
obstruct all the channels through a reef, except those kept open by
discharging the water, which during high tide and the greater part of
each ebb is thrown over its circumference. Several facts indicate that
a considerable quantity of sediment is likewise discharged through
these channels; and Captain Moresby informs me that he has observed,
during the change of the monsoon, the sea discoloured to a distance off
the entrances into the Maldiva and Chagos atolls. This, probably, would
check the growth of the coral in them, far more effectually than a mere
current of water. In the many small atolls without any channel, these
causes have not prevented the entire ring attaining the surface. The
channels, like the submerged and effaced parts of the reef, very
generally though not invariably occur on the leeward side of the atoll,
or on that side, according to Beechey,[21] which, from running in the
same direction with the prevalent wind, is not fully exposed to it.
Passages between the islets on the reef, through which boats can pass
at high water, must not be confounded with ship-channels, by which the
annular reef itself is breached. The passages between the islets occur,
of course, on the windward as well as on the leeward side; but they are
more frequent and broader to leeward, owing to the lesser dimensions of
the islets on that side.
[20] “Principles of Geology,” volume iii., page 289.
[21] Beechey’s “Voyage,” 4to edition, volume i., page 189.
At Keeling atoll the shores of the lagoon shelve gradually, where the
bottom is of sediment, and irregularly or abruptly where there are
coral-reefs; but this is by no means the universal structure in other
atolls. Chamisso,[22] speaking in general terms of the lagoons in the
Marshall atolls, says the lead generally sinks “from a depth of two or
three fathoms to twenty or twenty-four, and you may pursue a line in
which on one side of the boat you may see the bottom, and on the other
the azure-blue deep water.” The shores of the lagoon-like channel
within the barrier-reef at Vanikoro have a similar structure. Captain
Beechey has described a modification of this structure (and he believes
it is not uncommon) in two atolls in the Low Archipelago, in which the
shores of the lagoon descend by a few, broad, slightly inclined ledges
or steps: thus at Matilda atoll,[23] the great exterior reef, the
surface of which is gently inclined towards and beneath the surface of
the lagoon, ends abruptly in a little cliff three fathoms deep; at its
foot, a ledge forty yards wide extends, shelving gently inwards like
the surface-reef, and terminated by a second little cliff five fathoms
deep; beyond this, the bottom of the lagoon slopes to twenty fathoms,
which is the average depth of its centre. These ledges seem to be
formed of coral-rock; and Captain Beechey says that the lead often
descended several fathoms through holes in them. In some atolls, all
the coral reefs or knolls in the lagoon come to the surface at low
water; in other cases of rarer occurrence, all lie at nearly the same
depth beneath it, but most frequently they are quite irregular,—some
with perpendicular, some with sloping sides,—some rising to the
surface, and others lying at all intermediate depths from the bottom
upwards. I cannot, therefore, suppose that the union of such reefs
could produce even one uniformly sloping ledge, and much less two or
three, one beneath the other, and each terminated by an abrupt wall. At
Matilda Island, which offers the best example of the step-like
structure, Captain Beechey observes that the coral-knolls within the
lagoon are quite irregular in their height. We shall hereafter see that
the theory which accounts for the ordinary form of atolls, apparently
includes this occasional peculiarity in their structure.
[22] Kotzebue’s “First Voyage,” volume iii., page 142.
[23] Beechey’s “Voyage,” 4to edition, volume i, page 160. At
Whitsunday Island the bottom of the lagoon slopes gradually towards
the centre, and then deepens suddenly, the edge of the bank being
nearly perpendicular. This bank is formed of coral and dead shells.
In the midst of a group of atolls, there sometimes occur small, flat,
very low islands of coral formation, which probably once included a
lagoon, since filled up with sediment and coral-reefs. Captain Beechey
entertains no doubt that this has been the case with the two small
islands, which alone of thirty-one surveyed by him in the Low
Archipelago, did not contain lagoons. Romanzoff Island (in lat. 15° S.)
is described by Chamisso[24]
as formed by a dam of madreporitic rock inclosing a flat space, thinly
covered with trees, into which the sea on the leeward side occasionally
breaks. North Keeling atoll appears to be in a rather less forward
stage of conversion into land; it consists of a horse-shoe shaped strip
of land surrounding a muddy flat, one mile in its longest axis, which
is covered by the sea only at high water. When describing South Keeling
atoll, I endeavoured to show how slow the final process of filling up a
lagoon must be; nevertheless, as all causes do tend to produce this
effect, it is very remarkable that not one instance, as I believe, is
known of a moderately sized lagoon being filled up even to the low
water-line at spring-tides, much less of such a one being converted
into land. It is, likewise, in some degree remarkable, how few atolls,
except small ones, are surrounded by a single linear strip of land,
formed by the union of separate islets. We cannot suppose that the many
atolls in the Pacific and Indian Oceans all have had a late origin, and
yet should they remain at their present level, subjected only to the
action of the sea and to the growing powers of the coral, during as
many centuries as must have elapsed since any of the earlier tertiary
epochs, it cannot, I think, be doubted that their lagoons and the
islets on their reef, would present a totally different appearance from
what they now do. This consideration leads to the suspicion that some
renovating agency (namely subsidence) comes into play at intervals, and
perpetuates their original structure.
[24] Kotzebue’s “First Voyage,” volume iii., page 221.
SECTION III.—ATOLLS OF THE MALDIVA ARCHIPELAGO—GREAT CHAGOS BANK.
Maldiva Archipelago.—Ring-formed reefs, marginal and central.—Great
depths in the lagoons of the southern atolls.—Reefs in the lagoons all
rising to the surface.—Position of islets and breaches in the reefs,
with respect to the prevalent winds and action of the
waves.—Destruction of islets.—Connection in the position and submarine
foundation of distinct atolls.—The apparent disseverment of large
atolls.—The Great Chagos Bank.—Its submerged condition and
extraordinary structure.
Although occasional references have been made to the Maldiva atolls,
and to the banks in the Chagos group, some points of their structure
deserve further consideration. My description is derived from an
examination of the admirable charts lately published from the survey of
Captain Moresby and Lieutenant Powell, and more especially from
information which Captain Moresby has communicated to me in the kindest
manner.
The Maldiva Archipelago is 470 miles in length, with an average breadth
of about 50 miles. The form and dimensions of the atolls, and their
singular position in a double line, may be seen, but not well, in the
greatly reduced chart (Figure 6) in Plate II. The dimensions of the
longest atoll in the group (called by the double name of
Milla-dou-Madou and Tilla-dou-Matte) have already been given; it is 88
miles in a medial and slightly curved line, and is less than 20 miles
in its broadest part. Suadiva, also, is a noble atoll, being 44 miles
across in one direction, and 34 in another, and the great included
expanse of water has a depth of between 250 and 300 feet. The smaller
atolls in this group differ in no respect from ordinary ones; but the
larger ones are remarkable from being breached by numerous deep-water
channels leading into the lagoon; for instance, there are 42 channels,
through which a ship could enter the lagoon of Suadiva. In the three
southern large atolls, the separate portions of reef between these
channels have the ordinary structure, and are linear; but in the other
atolls, especially the more northern ones, these portions are ring-
formed, like miniature atolls. Other ring-formed reefs rise out of the
lagoons, in the place of those irregular ones which ordinarily occur
there. In the reduction of the chart of Mahlos Mahdoo (Plate II.,
Figure 4), it was not found easy to define the islets and the little
lagoons within each reef, so that the ring-formed structure is very
imperfectly shown; in the large published charts of Tilla-dou-Matte,
the appearance of these rings, from standing further apart from each
other, is very remarkable. The rings on the margin are generally
elongated; many of them are three, and some even five miles, in
diameter; those within the lagoon are usually smaller, few being more
than two miles across, and the greater number rather less than one. The
depth of the little lagoon within these small annular reefs is
generally from five to seven fathoms, but occasionally more; and in Ari
atoll many of the central ones are twelve, and some even more than
twelve fathoms deep. These rings rise abruptly from the platform or
bank, on which they are placed; their outer margin is invariably
bordered by living coral[25] within which there is a flat surface of
coral rock; of this flat, sand and fragments have in many cases
accumulated and been converted into islets, clothed with vegetation. I
can, in fact, point out no essential difference between these little
ring-formed reefs (which, however, are larger, and contain deeper
lagoons than many true atolls that stand in the open sea), and the most
perfectly characterised atolls, excepting that the ring-formed reefs
are based on a shallow foundation, instead of on the floor of the open
sea, and that instead of being scattered irregularly, they are grouped
closely together on one large platform, with the marginal rings
arranged in a rudely formed circle.
[25] Captain Moresby informs me that _Millepora complanata_ is one of
the commonest kinds on the outer margin, as it is at Keeling atoll.
The perfect series which can be traced from portions of simple linear
reef, to others including long linear lagoons, and from these again to
oval or almost circular rings, renders it probable that the latter are
merely modifications of the linear or normal state. It is conformable
with this view, that the ring-formed reefs on the margin, even where
most perfect and standing furthest apart, generally have their longest
axes directed in the line which the reef would have held, if the atoll
had been bounded by an ordinary wall. We may also infer that the
central ring-formed reefs are modifications of those irregular ones,
which are found in the lagoons of all common atolls. It appears from
the charts on a large scale, that the ring-like structure is contingent
on the marginal channels or breaches being wide; and, consequently, on
the whole interior of the atoll being freely exposed to the waters of
the open sea. When the channels are narrow or few in number, although
the lagoon be of great size and depth (as in Suadiva), there are no
ring-formed reefs; where the channels are somewhat broader, the
marginal portions of reef, and especially those close to the larger
channels, are ring-formed, but the central ones are not so; where they
are broadest, almost every reef throughout the atoll is more or less
perfectly ring-formed. Although their presence is thus contingent on
the openness of the marginal channels, the theory of their formation,
as we shall hereafter see, is included in that of the parent atolls, of
which they form the separate portions.
The lagoons of all the atolls in the southern part of the Archipelago
are from ten to twenty fathoms deeper than those in the northern part.
This is well exemplified in the case of Addoo, the southernmost atoll
in the group, for although only nine miles in its longest diameter, it
has a depth of thirty-nine fathoms, whereas all the other small atolls
have comparatively shallow lagoons; I can assign no adequate cause for
this difference in depth. In the central and deepest part of the
lagoons, the bottom consists, as I am informed by Captain Moresby, of
stiff clay (probably a calcareous mud); nearer the border it consists
of sand, and in the channels through the reef, of hard sand-banks,
sandstone, conglomerate rubble, and a little live coral. Close outside
the reef and the line joining its detached portions (where intersected
by many channels), the bottom is sandy, and it slopes abruptly into
unfathomable depths. In most lagoons the depth is considerably greater
in the centre than in the channels; but in Tilla-dou-Matte, where the
marginal ring-formed reefs stand far apart, the same depth is carried
across the entire atoll, from the deep-water line on one side to that
on the other. I cannot refrain from once again remarking on the
singularity of these atolls,—a great sandy and generally concave disc
rises abruptly from the unfathomable ocean, with its central expanse
studded and its border symmetrically fringed with oval basins of
coral-rock, just lipping the surface of the sea, sometimes clothed with
vegetation, and each containing a little lake of clear water!
In the southern Maldiva atolls, of which there are nine large ones, all
the small reefs within the lagoons come to the surface, and are dry at
low water spring-tides; hence in navigating them, there is no danger
from submarine banks. This circumstance is very remarkable, as within
some atolls, for instance those of the neighbouring Chagos group, not a
single reef comes to the surface, and in most other cases a few only
do, and the rest lie at all intermediate depths from the bottom
upwards. When treating of the growth of coral I shall again refer to
this subject.
Although in the neighbourhood of the Maldiva Archipelago the winds,
during the monsoons, blow during nearly an equal time from opposite
quarters, and although, as I am informed by Captain Moresby, the
westerly winds are the strongest, yet the islets are almost all placed
on the eastern side of the northern atolls, and on the south-eastern
side of the southern atolls. That the formation of the islets is due to
detritus thrown up from the outside, as in the ordinary manner, and not
from the interior of the lagoons, may, I think be safely inferred from
several considerations, which it is hardly worth while to detail. As
the easterly winds are not the strongest, their action probably is
aided by some prevailing swell or current.
In groups of atolls, exposed to a trade-wind, the ship-channels into
the lagoons are almost invariably situated on the leeward or less
exposed side of the reef, and the reef itself is sometimes either
wanting there, or is submerged. A strictly analogous, but different
fact, may be observed at the Maldiva atolls—namely, that where two
atolls stand in front of each other, the breaches in the reef are the
most numerous on their near, and therefore less exposed, sides. Thus on
the near sides of Ari and the two Nillandoo atolls, which face S. Māle,
Phaleedoo, and Moloque atolls, there are seventy-three deep-water
channels, and only twenty-five on their outer sides; on the near side
of the three latter named atolls there are fifty- six openings, and
only thirty-seven on their outsides. It is scarcely possible to
attribute this difference to any other cause than the somewhat
different action of the sea on the two sides, which would ensue from
the protection afforded by the two rows of atolls to each other. I may
here remark that in most cases, the conditions favourable to the
greater accumulation of fragments on the reef and to its more perfect
continuity on one side of the atoll than on the other, have concurred,
but this has not been the case with the Maldivas; for we have seen that
the islets are placed on the eastern or south-eastern sides, whilst the
breaches in the reef occur indifferently on any side, where protected
by an opposite atoll. The reef being more continuous on the outer and
more exposed sides of those atolls which stand near each other, accords
with the fact, that the reef of the southern atolls is more continuous
than that of the northern ones; for the former, as I am informed by
Captain Moresby, are more constantly exposed than the northern atolls
to a heavy surf.
The date of the first formation of some of the islets in this
Archipelago is known to the inhabitants; on the other hand, several
islets, and even some of those which are believed to be very old, are
now fast wearing away. The work of destruction has, in some instances,
been completed in ten years. Captain Moresby found on one water-washed
reef the marks of wells and graves, which were excavated when it
supported an islet. In South Nillandoo atoll, the natives say that
three of the islets were formerly larger: in North Nillandoo there is
one now being washed away; and in this latter atoll Lieutenant Prentice
found a reef, about six hundred yards in diameter, which the natives
positively affirmed was lately an island covered with cocoa-nut trees.
It is now only partially dry at low water spring-tides, and is (in
Lieutenant Prentice’s words) “entirely covered with live coral and
madrepore.” In the northern part, also, of the Maldiva Archipelago and
in the Chagos group, it is known that some of the islets are
disappearing. The natives attribute these effects to variations in the
currents of the sea. For my own part I cannot avoid suspecting that
there must be some further cause, which gives rise to such a cycle of
change in the action of the currents of the great and open ocean.
Several of the atolls in this Archipelago are so related to each other
in form and position, that at the first glance one is led to suspect
that they have originated in the disseverment of a single one. Māle
consists of three perfectly characterised atolls, of which the shape
and relative position are such, that a line drawn closely round all
three, gives a symmetrical figure; to see this clearly, a larger chart
is required than that of the Archipelago in Plate II.; the channel
separating the two northern Māle atolls is only little more than a mile
wide, and no bottom was found in it with 100 fathoms. Powell’s Island
is situated at the distance of two miles and a half off the northern
end of Mahlos Mahdoo (see Figure 4, Plate II.), at the exact point
where the two sides of the latter, if prolonged, would meet; no bottom,
however, was found in the channel with 200 fathoms; in the wider
channel between Horsburgh atoll and the southern end of Mahlos Mahdoo,
no bottom was found with 250 fathoms. In these and similar cases, the
relation consists only in the form and position of the atolls. But in
the channel between the two Nillandoo atolls, although three miles and
a quarter wide, soundings were struck at the depth of 200 fathoms; the
channel between Ross and Ari atolls is four miles wide, and only 150
fathoms deep. Here then we have, besides the relation of form, a
submarine connection. The fact of soundings having been obtained
between two separate and perfectly characterised atolls is in itself
interesting, as it has never, I believe, been effected in any of the
many other groups of atolls in the Pacific and Indian seas. In
continuing to trace the connection of adjoining atolls, if a hasty
glance be taken at the chart (Figure 4., Plate II.) of Mahlos Mahdoo,
and the line of unfathomable water be followed, no one will hesitate to
consider it as one atoll. But a second look will show that it is
divided by a bifurcating channel, of which the northern arm is about
one mile and three-quarters in width, with an average depth of 125
fathoms, and the southern one three-quarters of a mile wide, and rather
less deep. These channels resemble in the slope of their sides and
general form, those which separate atolls in every respect distinct;
and the northern arm is wider than that dividing two of the Māle
atolls. The ring-formed reefs on the sides of this bifurcating channel
are elongated, so that the northern and southern portions of Mahlos
Mahdoo may claim, as far as their external outline is concerned, to be
considered as distinct and perfect atolls. But the intermediate
portion, lying in the fork of the channel, is bordered by reefs less
perfect than those which surround any other atoll in the group of
equally small dimensions. Mahlos Mahdoo, therefore, is in every respect
in so intermediate a condition, that it may be considered either as a
single atoll nearly dissevered into three portions, or as three atolls
almost perfect and intimately connected. This is an instance of a very
early stage of the apparent disseverment of an atoll, but a still
earlier one in many respects is exhibited at Tilla-dou- Matte. In one
part of this atoll, the ring-formed reefs stand so far apart from each
other, that the inhabitants have given different names to the northern
and southern halves; nearly all the rings, moreover, are so perfect and
stand so separate, and the space from which they rise is so level and
unlike a true lagoon, that we can easily imagine the conversion of this
one great atoll, not into two or three portions, but into a whole group
of miniature atolls. A perfect series such as we have here traced,
impresses the mind with an idea of actual change; and it will hereafter
be seen, that the theory of subsidence, with the upward growth of the
coral, modified by accidents of probable occurrence, will account for
the occasional disseverment of large atolls.
The Great Chagos bank alone remains to be described. In the Chagos
group there are some ordinary atolls, some annular reefs rising to the
surface but without any islets on them, and some atoll-formed banks,
either quite submerged, or nearly so. Of the latter, the Great Chagos
Bank is much the largest, and differs in its structure from the others:
a plan of it is given in Plate II., Figure 1, in which, for the sake of
clearness, I have had the parts under ten fathoms deep finely shaded:
an east and west vertical section is given in Figure 2, in which the
vertical scale has been necessarily exaggerated. Its longest axis is
ninety nautical miles, and another line drawn at right angles to the
first, across the broadest part, is seventy. The central part consists
of a level muddy flat, between forty and fifty fathoms deep, which is
surrounded on all sides, with the exception of some breaches, by the
steep edges of a set of banks, rudely arranged in a circle. These banks
consist of sand, with a very little live coral; they vary in breadth
from five to twelve miles, and on an average lie about sixteen fathoms
beneath the surface; they are bordered by the steep edges of a third
narrow and upper bank, which forms the rim to the whole. This rim is
about a mile in width, and with the exception of two or three spots
where islets have been formed, is submerged between five and ten
fathoms. It consists of smooth hard rock, covered with a thin layer of
sand, but with scarcely any live coral; it is steep on both sides, and
outwards slopes abruptly into unfathomable depths. At the distance of
less than half a mile from one part, no bottom was found with 190
fathoms; and off another point, at a somewhat greater distance, there
was none with 210 fathoms. Small steep-sided banks or knolls, covered
with luxuriantly growing coral, rise from the interior expanse to the
same level with the external rim, which, as we have seen, is formed
only of dead rock. It is impossible to look at the plan (Figure 1,
Plate II.), although reduced to so small a scale, without at once
perceiving that the Great Chagos Bank is, in the words of Captain
Moresby,[26] “nothing more than a half-drowned atoll.” But of what
great dimensions, and of how extraordinary an internal structure? We
shall hereafter have to consider both the cause of its submerged
condition, a state common to other banks in the group, and the origin
of the singular submarine terraces, which bound the central expanse:
these, I think, it can be shown, have resulted from a cause analogous
to that which has produced the bifurcating channel across Mahlos
Mahdoo.
[26] This officer has had the kindness to lend me an excellent MS.
account of the Chagos Islands; from this paper, from the published
charts, and from verbal information communicated to me by Captain
Moresby, the above account of the Great Chagos Bank is taken.
CHAPTER II.
BARRIER REEFS.
Closely resemble in general form and structure atoll-reefs.—Width and
depth of the lagoon-channels.—Breaches through the reef in front of
valleys, and generally on the leeward side.—Checks to the filling up of
the lagoon-channels.—Size and constitution of the encircled islands.—
Number of islands within the same reef.—Barrier-reefs of New Caledonia
and Australia.—Position of the reef relative to the slope of the
adjoining land.—Probable great thickness of barrier-reefs.
The term “barrier” has been generally applied to that vast reef which
fronts the N.E. shore of Australia, and by most voyagers likewise to
that on the western coast of New Caledonia. At one time I thought it
convenient thus to restrict the term, but as these reefs are similar in
structure, and in position relatively to the land, to those, which,
like a wall with a deep moat within, encircle many smaller islands, I
have classed them together. The reef, also, on the west coast of New
Caledonia, circling round the extremities of the island, is an
intermediate form between a small encircling reef and the Australian
barrier, which stretches for a thousand miles in nearly a straight
line.
The geographer Balbi has in effect described those barrier-reefs, which
encircle moderately sized islands, by calling them atolls with high
land rising from within their central expanse. The general resemblance
between the reefs of the barrier and atoll classes may be seen in the
small, but accurately reduced charts on Plate I.,[1] and this
resemblance can be further shown to extend to every part of the
structure. Beginning with the outside of the reef; many scattered
soundings off Gambier, Oualan, and some other encircled islands, show
that close to the breakers there exists a narrow shelving margin,
beyond which the ocean becomes suddenly unfathomable; but off the west
coast of New Caledonia, Captain Kent[2] found no bottom with 150
fathoms, at two ships’ length from the reef; so that the slope here
must be nearly as precipitous as off the Maldiva atolls.
[1] The authorities from which these charts have been reduced,
together with some remarks on them and descriptive of the Plates, are
given separately.
[2] Dalrymple, “Hydrog. Mem.” volume iii.
I can give little information regarding the kinds of corals which live
on the outer margin. When I visited the reef at Tahiti, although it was
low water, the surf was too violent for me to see the living masses;
but, according to what I heard from some intelligent native chiefs,
they resemble in their rounded and branchless forms, those on the
margin of Keeling atoll. The extreme verge of the reef, which was
visible between the breaking waves at low water, consisted of a
rounded, convex, artificial-like breakwater, entirely coated with
Nulliporæ, and absolutely similar to that which I have described at
Keeling atoll. From what I heard when at Tahiti, and from the writings
of the Revs. W. Ellis and J. Williams, I conclude that this peculiar
structure is common to most of the encircled islands of the Society
Archipelago. The reef within this mound or breakwater, has an extremely
irregular surface, even more so than between the islets on the reef of
Keeling atoll, with which alone (as there are no islets on the reef of
Tahiti) it can properly be compared. At Tahiti, the reef is very
irregular in width; but round many other encircled islands, for
instance, Vanikoro or Gambier Islands (Figures 1 and 8, Plate I.), it
is quite as regular, and of the same average width, as in true atolls.
Most barrier-reefs on the inner side slope irregularly into the
lagoon-channel (as the space of deep water separating the reef from the
included land may be called), but at Vanikoro the reef slopes only for
a short distance, and then terminates abruptly in a submarine wall,
forty feet high,—a structure absolutely similar to that described by
Chamisso in the Marshall atolls.
In the Society Archipelago, Ellis[3] states, that the reefs generally
lie at the distance of from one to one and a half miles, and,
occasionally, even at more than three miles, from the shore. The
central mountains are generally bordered by a fringe of flat, and often
marshy, alluvial land, from one to four miles in width. This fringe
consists of coral-sand and detritus thrown up from the lagoon-channel,
and of soil washed down from the hills; it is an encroachment on the
channel, analogous to that low and inner part of the islets in many
atolls which is formed by the accumulation of matter from the lagoon.
At Hogoleu (Figure 2, Plate I.), in the Caroline Archipelago,[4] the
reef on the south side is no less than twenty miles; on the east side,
five; and on the north side, fourteen miles from the encircled high
islands.
[3] Consult, on this and other points, the “Polynesian Researches,” by
the Rev. W. Ellis, an admirable work, full of curious information.
[4] See “Hydrographical Mem.” and the “Atlas of the Voyage of the
_Astrolabe_,” by Captain Dumont D’Urville, page 428.
The lagoon channels may be compared in every respect with true lagoons.
In some cases they are open, with a level bottom of fine sand; in
others they are choked up with reefs of delicately branched corals,
which have the same general character as those within the Keeling
atoll. These internal reefs either stand separately, or more commonly
skirt the shores of the included high islands. The depth of the
lagoon-channel round the Society Islands varies from two or three to
thirty fathoms; in Cook’s[5] chart of Ulieta, however, there is one
sounding laid down of forty-eight fathoms; at Vanikoro there are
several of fifty-four and one of fifty-six and a half fathoms
(English), a depth which even exceeds by a little that of the interior
of the great Maldiva atolls. Some barrier-reefs have very few islets on
them; whilst others are surmounted by numerous ones; and those round
part of Bolabola (Plate I., Figure 5) form a single linear strip. The
islets first appear either on the angles of the reef, or on the sides
of the breaches through it, and are generally most numerous on the
windward side. The reef to leeward retaining its usual width, sometimes
lies submerged several fathoms beneath the surface; I have already
mentioned Gambier Island as an instance of this structure. Submerged
reefs, having a less defined outline, dead, and covered with sand, have
been observed (see Appendix) off some parts of Huaheine and Tahiti. The
reef is more frequently breached to leeward than to windward; thus I
find in Krusenstern’s “Memoir on the Pacific,” that there are passages
through the encircling reef on the leeward side of each of the seven
Society Islands, which possess ship-harbours; but that there are
openings to windward through the reef of only three of them. The
breaches in the reef are seldom as deep as the interior lagoon-like
channel; they generally occur in front of the main valleys, a
circumstance which can be accounted for, as will be seen in the fourth
chapter, without much difficulty. The breaches being situated in front
of the valleys, which descend indifferently on all sides, explains
their more frequent occurrence through the windward side of
barrier-reefs than through the windward side of atolls,—for in atolls
there is no included land to influence the position of the breaches.
[5] See the chart in volume i. of Hawkesworth’s 4to edition of “Cook’s
First Voyage.”
It is remarkable, that the lagoon-channels round mountainous islands
have not in every instance been long ago filled up with coral and
sediment; but it is more easily accounted for than appears at first
sight. In cases like that of Hogoleu and the Gambier Islands, where a
few small peaks rise out of a great lagoon, the conditions scarcely
differ from those of an atoll, and I have already shown, at some
length, that the filling up of a true lagoon must be an extremely slow
process. Where the channel is narrow, the agency, which on unprotected
coasts is most productive of sediment, namely the force of the
breakers, is here entirely excluded, and the reef being breached in the
front of the main valleys, much of the finer mud from the rivers must
be transported into the open sea. As a current is formed by the water
thrown over the edge of atoll-formed reefs, which carries sediment with
it through the deep-water breaches, the same thing probably takes place
in barrier-reefs, and this would greatly aid in preventing the
lagoon-channel from being filled up. The low alluvial border, however,
at the foot of the encircled mountains, shows that the work of filling
up is in progress; and at Maura (Plate I., Figure 6), in the Society
group, it has been almost effected, so that there remains only one
harbour for small craft.
If we look at a set of charts of barrier-reefs, and leave out in
imagination the encircled land, we shall find that, besides the many
points already noticed of resemblance, or rather of identity in
structure with atolls, there is a close general agreement in form,
average dimensions, and grouping. Encircling barrier-reefs, like
atolls, are generally elongated, with an irregularly rounded, though
sometimes angular outline. There are atolls of all sizes, from less
than two miles in diameter to sixty miles (excluding Tilla-dou-Matte,
as it consists of a number of almost independent atoll-formed reefs);
and there are encircling barrier-reefs from three miles and a half to
forty-six miles in diameter,—Turtle Island being an instance of the
former, and Hogoleu of the latter. At Tahiti the encircled island is
thirty-six miles in its longest axis, whilst at Maurua it is only a
little more than two miles. It will be shown, in the last chapter in
this volume, that there is the strictest resemblance in the grouping of
atolls and of common islands, and consequently there must be the same
resemblance in the grouping of atolls and of encircling barrier-reefs.
The islands lying within reefs of this class, are of very various
heights. Tahiti[6] is 7,000 feet; Maurua about 800; Aitutaki 360, and
Manouai only 50. The geological nature of the included land varies: in
most cases it is of ancient volcanic origin, owing apparently to the
fact that islands of this nature are most frequent within all great
seas; some, however, are of madreporitic limestone, and others of
primary formation, of which latter kind New Caledonia offers the best
example. The central land consists either of one island, or of several:
thus, in the Society group, Eimeo stands by itself; while Taha and
Raiatea (Figure 3, Plate I.), both moderately large islands of nearly
equal size, are included in one reef. Within the reef of the Gambier
group there are four large and some smaller islands (Figure 8, Plate
I.); within that of Hogoleu (Figure 2, Plate I.) nearly a dozen small
islands are scattered over the expanse of one vast lagoon.
[6] The height of Tahiti is given from Captain Beechey; Maurua from
Mr. F.D. Bennett (“Geograph. Journ.” volume viii., page 220); Aitutaki
from measurements made on board the _Beagle;_ and Manouai or Harvey
Island, from an estimate by the Rev. J. Williams. The two latter
islands, however, are not in some respects well characterised examples
of the encircled class.
After the details now given, it may be asserted that there is not one
point of essential difference between encircling barrier-reefs and
atolls: the latter enclose a simple sheet of water, the former encircle
an expanse with one or more islands rising from it. I was much struck
with this fact, when viewing, from the heights of Tahiti, the distant
island of Eimeo standing within smooth water, and encircled by a ring
of snow-white breakers. Remove the central land, and an annular reef
like that of an atoll in an early stage of its formation is left;
remove it from Bolabola, and there remains a circle of linear
coral-islets, crowned with tall cocoa-nut trees, like one of the many
atolls scattered over the Pacific and Indian Oceans.
The barrier-reefs of Australia and of New Caledonia deserve a separate
notice from their great dimensions. The reef on the west coast of New
Caledonia (Figure 5, Plate II.) is 400 miles in length; and for a
length of many leagues it seldom approaches within eight miles of the
shore; and near the southern end of the island, the space between the
reef and the land is sixteen miles in width. The Australian barrier
extends, with a few interruptions, for nearly a thousand miles; its
average distance from the land is between twenty and thirty miles; and
in some parts from fifty to seventy. The great arm of the sea thus
included, is from ten to twenty-five fathoms deep, with a sandy bottom;
but towards the southern end, where the reef is further from the shore,
the depth gradually increases to forty, and in some parts to more than
sixty fathoms. Flinders[7] has described the surface of this reef as
consisting of a hard white agglomerate of different kinds of coral,
with rough projecting points. The outer edge is the highest part; it is
traversed by narrow gullies, and at rare intervals is breached by
ship-channels. The sea close outside is profoundly deep; but, in front
of the main breaches, soundings can sometimes be obtained. Some low
islets have been formed on the reef.
[7] Flinders’ “Voyage to Terra Australis,” volume ii., page 88.
[Illustration]
1. VANIKORO, from the “Atlas of the Voyage of the _Astrolabe_,” by D.
D’Urville.
2. GAMBIER ISLAND, from Beechey.
3. MAURUA, from the “Atlas of the Voyage of the _Coquille_,” by
Duperrey.
The horizontal line is the level of the sea, from which on the right
hand a plummet descends, representing a depth of 200 fathoms, or 1,200
feet. The vertical shading shows the section of the land, and the
horizontal shading that of the encircling barrier-reef: from the
smallness of the scale, the lagoon-channel could not be represented.
AA.—Outer edge of the coral-reefs, where the sea breaks.
BB.—The shore of the encircled islands.)
There is one important point in the structure of barrier-reefs which
must here be considered. The accompanying diagrams represent north and
south vertical sections, taken through the highest points of Vanikoro,
Gambier, and Maurua Islands, and through their encircling reefs. The
scale both in the horizontal and vertical direction is the same,
namely, a quarter of an inch to a nautical mile. The height and width
of these islands is known; and I have attempted to represent the form
of the land from the shading of the hills in the large published
charts. It has long been remarked, even from the time of Dampier, that
considerable degree of relation subsists between the inclination of
that part of the land which is beneath water and that above it; hence
the dotted line in the three sections, probably, does not widely differ
in inclination from the actual submarine prolongation of the land. If
we now look at the outer edge of the reef (AA), and bear in mind that
the plummet on the right hand represents a depth of 1,200 feet, we must
conclude that the vertical thickness of these barrier coral-reefs is
very great.
I must observe that if the sections had been taken in any other
direction across these islands, or across other encircled islands,[8]
the result would have been the same. In the succeeding chapter it will
be shown that reef-building polypifers cannot flourish at great
depths,—for instance, it is highly improbable that they could exist at
a quarter of the depth represented by the plummet on the right hand of
the woodcut. Here there is a great _apparent_ difficulty—how were the
basal parts of these barrier-reef formed? It will, perhaps, occur to
some, that the actual reefs formed of coral are not of great thickness,
but that before their first growth, the coasts of these encircled
islands were deeply eaten into, and a broad but shallow submarine ledge
thus left, on the edge of which the coral grew; but if this had been
the case, the shore would have been invariably bounded by lofty cliffs,
and not have sloped down to the lagoon-channel, as it does in many
instances. On this view,[9] moreover, the cause of the reef springing
up at such a great distance from the land, leaving a deep and broad
moat within, remains altogether unexplained. A supposition of the same
nature, and appearing at first more probable is, that the reefs sprung
up from banks of sediment, which had accumulated round the shore
previously to the growth of the coral; but the extension of a bank to
the same distance round an unbroken coast, and in front of those deep
arms of the sea (as in Raiatea, see Plate II., Figure 3) which
penetrate nearly to the heart of some encircled islands, is exceedingly
improbable. And why, again, should the reef spring up, in some cases
steep on both sides like a wall, at a distance of two, three or more
miles from the shore, leaving a channel often between two hundred and
three hundred feet deep, and rising from a depth which we have reason
to believe is destructive to the growth of coral? An admission of this
nature cannot possibly be made. The existence, also, of the deep
channel, utterly precludes the idea of the reef having grown outwards,
on a foundation slowly formed on its outside, by the accumulation of
sediment and coral detritus. Nor, again, can it be asserted, that the
reef-building corals will not grow, excepting at a great distance from
the land; for, as we shall soon see, there is a whole class of reefs,
which take their name from growing closely attached (especially where
the sea is deep) to the beach. At New Caledonia (see Plate II., Figure
5) the reefs which run in front of the west coast are prolonged in the
same line 150 miles beyond the northern extremity of the island, and
this shows that some explanation, quite different from any of those
just suggested, is required. The continuation of the reefs on each side
of the submarine prolongation of New Caledonia, is an exceedingly
interesting fact, if this part formerly existed as the northern
extremity of the island, and before the attachment of the coral had
been worn down by the action of the sea, or if it originally existed at
its present height, with or without beds of sediment on each flank, how
can we possibly account for the reefs, not growing on the crest of this
submarine portion, but fronting its sides, in the same line with the
reefs which front the shores of the lofty island? We shall hereafter
see, that there is one, and I believe only one, solution of this
difficulty.
[8] In the fifth chapter an east and west section across the Island of
Bolabola and its barrier-reefs is given, for the sake of illustrating
another point. The unbroken line in it (woodcut No. 5) is the section
referred to. The scale is .57 of an inch to a mile; it is taken from
the “Atlas of the Voyage of the _Coquille_,” by Duperrey. The depth of
the lagoon-channel is exaggerated.
[9] The Rev. D. Tyerman and Mr. Bennett (“Journal of Voyage and
Travels,” volume i., page 215) have briefly suggested this explanation
of the origin of the encircling reefs of the Society Islands.
One other supposition to account for the position of encircling
barrier-reefs remains, but it is almost too preposterous to be
mentioned; namely, that they rest on enormous submarine craters,
surrounding the included islands. When the size, height, and form of
the islands in the Society group are considered, together with the fact
that all are thus encircled, such a notion will be rejected by almost
every one. New Caledonia, moreover, besides its size, is composed of
primitive formations, as are some of the Comoro Islands;[10] and
Aitutaki consists of calcareous rock. We must, therefore, reject these
several explanations, and conclude that the vertical thickness of
barrier-reefs, from their outer edges to the foundation on which they
rest (from _AA_ in the section to the dotted lines) is really great;
but in this, there is no difficulty, for it is not necessary to suppose
that the coral has sprung up from an immense depth, as will be evident
when the theory of the upward growth of coral-reefs, during the slow
subsidence of their foundation, is discussed.
[10] I have been informed that this is the case by Dr. Allan of
Forres, who has visited this group.
CHAPTER III.
FRINGING OR SHORE-REEFS.
Reefs of Mauritius.—Shallow channel within the reef.—Its slow filling
up.—Currents of water formed within it.—Upraised reefs.—Narrow
fringing-reefs in deep seas.—Reefs on the coast of East Africa and of
Brazil.—Fringing-reefs in very shallow seas, round banks of sediment
and on worn-down islands.—Fringing-reefs affected by currents of the
sea.— Coral coating the bottom of the sea, but not forming reefs.
Fringing-reefs, or, as they have been called by some voyagers,
shore-reefs, whether skirting an island or part of a continent, might
at first be thought to differ little, except in generally being of less
breadth, from barrier-reefs. As far as the superficies of the actual
reef is concerned this is the case; but the absence of an interior
deep-water channel, and the close relation in their horizontal
extension with the probable slope beneath the sea of the adjoining
land, present essential points of difference.
The reefs which fringe the island of Mauritius offer a good example of
this class. They extend round its whole circumference, with the
exception of two or three parts,[1] where the coast is almost
precipitous, and where, if as is probable the bottom of the sea has a
similar inclination, the coral would have no foundation on which to
become attached. A similar fact may sometimes be observed even in reefs
of the barrier class, which follow much less closely the outline of the
adjoining land; as, for instance, on the south-east and precipitous
side of Tahiti, where the encircling reef is interrupted. On the
western side of the Mauritius, which was the only part I visited, the
reef generally lies at the distance of about half a mile from the
shore; but in some parts it is distant from one to two, and even three
miles. But even in this last case, as the coast-land is gently inclined
from the foot of the mountains to the sea-beach, and as the soundings
outside the reef indicate an equally gentle slope beneath the water,
there is no reason for supposing that the basis of the reef, formed by
the prolongation of the strata of the island, lies at a greater depth
than that at which the polypifers could begin constructing the reef.
Some allowance, however, must be made for the outward extension of the
corals on a foundation of sand and detritus, formed from their own
wear, which would give to the reef a somewhat greater vertical
thickness, than would otherwise be possible.
[1] This fact is stated on the authority of the Officier du Roi, in
his extremely interesting “Voyage à l’Isle de France,” undertaken in
1768. According to Captain Carmichael (Hooker’s “Bot. Misc.” volume
ii., page 316) on one part of the coast there is a space for sixteen
miles without a reef.
The outer edge of the reef on the western or leeward side of the island
is tolerably well defined, and is a little higher than any other part.
It chiefly consists of large strongly branched corals, of the genus
Madrepora, which also form a sloping bed some way out to sea: the kinds
of coral growing in this part will be described in the ensuing chapter.
Between the outer margin and the beach, there is a flat space with a
sandy bottom and a few tufts of living coral; in some parts it is so
shallow, that people, by avoiding the deeper holes and gullies, can
wade across it at low water; in other parts it is deeper, seldom
however exceeding ten or twelve feet, so that it offers a safe coasting
channel for boats. On the eastern and windward side of the island,
which is exposed to a heavy surf, the reef was described to me as
having a hard smooth surface, very slightly inclined inwards, just
covered at low-water, and traversed by gullies; it appears to be quite
similar in structure to the reefs of the barrier and atoll classes.
The reef of Mauritius, in front of every river and streamlet, is
breached by a straight passage: at Grand Port, however, there is a
channel like that within a barrier-reef; it extends parallel to the
shore for four miles, and has an average depth of ten or twelve
fathoms; its presence may probably be accounted for by two rivers which
enter at each end of the channel, and bend towards each other. The fact
of reefs of the fringing class being always breached in front of
streams, even of those which are dry during the greater part of the
year, will be explained, when the conditions unfavourable to the growth
of coral are considered. Low coral-islets, like those on barrier-reefs
and atolls, are seldom formed on reefs of this class, owing apparently
in some cases to their narrowness, and in others to the gentle slope of
the reef outside not yielding many fragments to the breakers. On the
windward side, however, of the Mauritius, two or three small islets
have been formed.
It appears, as will be shown in the ensuing chapter, that the action of
the surf is favourable to the vigorous growth of the stronger corals,
and that sand or sediment, if agitated by the waves, is injurious to
them. Hence it is probable that a reef on a shelving shore, like that
of Mauritius, would at first grow up, not attached to the actual beach,
but at some little distance from it; and the corals on the outer margin
would be the most vigorous. A shallow channel would thus be formed
within the reef, and as the breakers are prevented acting on the shores
of the island, and as they do not ordinarily tear up many fragments
from the outside, and as every streamlet has its bed prolonged in a
straight line through the reef, this channel could be filled up only
very slowly with sediment. But a beach of sand and of fragments of the
smaller kinds of coral seems, in the case of Mauritius, to be slowly
encroaching on the shallow channel. On many shelving and sandy coasts,
the breakers tend to form a bar of sand a little way from the beach,
with a slight increase of depth within it; for instance, Captain
Grey[2] states that the west coast of Australia, in latitude 24°, is
fronted by a sand bar about two hundred yards in width, on which there
is only two feet of water; but within it the depth increases to two
fathoms. Similar bars, more or less perfect, occur on other coasts. In
these cases I suspect that the shallow channel (which no doubt during
storms is occasionally obliterated) is scooped out by the flowing away
of the water thrown beyond the line, on which the waves break with the
greatest force. At Pernambuco a bar of hard sandstone,[3] which has the
same external form and height as a coral-reef, extends nearly parallel
to the coast; within this bar currents, apparently caused by the water
thrown over it during the greater part of each tide, run strongly, and
are wearing away its inner wall. From these facts it can hardly be
doubted, that within most fringing-reefs, especially within those lying
some distance from the land, a return stream must carry away the water
thrown over the outer edge; and the current thus produced, would tend
to prevent the channel being filled up with sediment, and might even
deepen it under certain circumstances. To this latter belief I am led,
by finding that channels are almost universally present within the
fringing-reefs of those islands which have undergone recent elevatory
movements; and this could hardly have been the case, if the conversion
of the very shallow channel into land had not been counteracted to a
certain extent.
[2] Captain Grey’s “Journal of Two Expeditions,” volume i. page 369.
[3] I have described this singular structure in the “London and
Edinburgh Phil. Mag.” October 1841.
A fringing-reef, if elevated in a perfect condition above the level of
the sea, ought to present the singular appearance of a broad dry moat
within a low mound. The author[4] of an interesting pedestrian tour
round the Mauritius, seems to have met with a structure of this kind:
he says “J’observai que là, où la mer étale, indépendamment des rescifs
du large, il y à terre _une espèce d’effoncement_ ou chemin couvert
naturel. On y pourrait mettre du canon,” etc. In another place he adds,
“Avant de passer le Cap, on remarque un gros banc de corail elévé de
plus de quinze pieds: c’est une espèce de rescif, que la mer abandonné,
il regne au pied une longue flaque d’eau, dont on pourrait faire un
bassin pour de petits vaisseaux.” But the margin of the reef, although
the highest and most perfect part, from being most exposed to the surf,
would generally during a slow rise of the land be either partially or
entirely worn down to that level, at which corals could renew their
growth on its upper edge. On some parts of the coast-land of Mauritius
there are little hillocks of coral-rock, which are either the last
remnants of a continuous reef, or of low islets formed on it. I
observed that two such hillocks between Tamarin Bay and the Great Black
River; they were nearly twenty feet high, about two hundred yards from
the present beach, and about thirty feet above its level. They rose
abruptly from a smooth surface, strewed with worn fragments of coral.
They consisted in their lower part of hard calcareous sandstone, and in
their upper of great blocks of several species of Astræa and Madrepora,
loosely aggregated; they were divided into irregular beds, dipping
seaward, in one hillock at an angle of 8°, and in the other at 18° I
suspect that the superficial parts of the reefs, which have been
upraised together with the islands they fringe, have generally been
much more modified by the wearing action of the sea, than those of
Mauritius.
[4] “Voyage à l’Isle de France, par un Officier du Roi,” part i.,
pages 192, 200.
Many islands[5] are fringed by reefs quite similar to those of
Mauritius; but on coasts where the sea deepens very suddenly the reefs
are much narrower, and their limited extension seems evidently to
depend on the high inclination of the submarine slope; a relation,
which, as we have seen, does not exist in reefs of the barrier class.
The fringing-reefs on steep coasts are frequently not more than from
fifty to one hundred yards in width; they have a nearly smooth, hard
surface, scarcely uncovered at low water, and without any interior
shoal channel, like that within those fringing-reefs, which lie at a
greater distance from the land. The fragments torn up during gales from
the outer margin are thrown over the reef on the shores of the island.
I may give as instances, Wateeo, where the reef is described by Cook as
being a hundred yards wide; and Mauti and Elizabeth Islands,[6] where
it is only fifty yards in width: the sea round these islands is very
deep.
[5] I may give Cuba, as another instance; Mr. Taylor (Loudon’s Mag. of
Nat. Hist. volume ix., page 449) has described a reef several miles in
length between Gibara and Vjaro, which extends parallel to the shore
at the distance of between half and the third part of a mile, and
encloses a space of shallow water, with a sandy bottom and tufts of
coral. Outside the edge of the reef, which is formed of great
branching corals, the depth is six and seven fathoms. This coast has
been upheaved at no very distant geological period.”
[6] Mauti is described by Lord Byron in the voyage of H.M.S. _Blonde_,
and Elizabeth Island by Captain Beechey.
Fringing-reefs, like barrier-reefs, both surround islands, and front
the shores of continents. In the charts of the eastern coast of Africa,
by Captain Owen, many extensive fringing-reefs are laid down; thus, for
a space of nearly forty miles, from latitude 1° 15′ to 1° 45′ S., a
reef fringes the shore at an average distance of rather more than one
mile, and therefore at a greater distance than is usual in reefs of
this class; but as the coast-land is not lofty, and as the bottom
shoals very gradually (the depth being only from eight to fourteen
fathoms at a mile and a half outside the reef), its extension thus far
from the land offers no difficulty. The external margin of this reef is
described, as formed of projecting points, within which there is a
space, from six to twelve feet deep, with patches of living coral on
it. At Mukdeesha (latitude 2° 1′ N.) “the port is formed,” it is
said,[7] “by a long reef extending eastward, four or five miles, within
which there is a narrow channel, with ten to twelve feet of water at
low spring-tides;” it lies at the distance of a quarter of a mile from
the shore. Again, in the plan of Mombas (latitude 4° S.), a reef
extends for thirty-six miles, at the distance of from half a mile to
one mile and a quarter from the shore; within it, there is a channel
navigable “for canoes and small craft,” between six and fifteen feet
deep: outside the reef the depth is about thirty fathoms at the
distance of nearly half a mile. Part of this reef is very symmetrical,
and has a uniform breadth of two hundred yards.
[7] Owen’s “Africa,” volume i., page 357, from which work the
foregoing facts are likewise taken.
The coast of Brazil is in many parts fringed by reefs. Of these, some
are not of coral formation; for instance, those near Bahia and in front
of Pernambuco; but a few miles south of this latter city, the reef
follows[8] so closely every turn of the shore, that I can hardly doubt
it is of coral; it runs at the distance of three-quarters of a mile
from the land, and within it the depth is from ten to fifteen feet. I
was assured by an intelligent pilot that at Ports Frances and Maceio,
the outer part of the reef consists of living coral, and the inner of a
white stone, full of large irregular cavities, communicating with the
sea. The bottom of the sea off the coast of Brazil shoals gradually to
between thirty and forty fathoms, at the distance of between nine and
ten leagues from the land.
[8] See Baron Roussin’s “Pilote du Brésil,” and accompanying
hydrographical memoir.
From the description now given, we must conclude that the dimensions
and structure of fringing-reefs depend entirely on the greater or less
inclination of the submarine slope, conjoined with the fact that
reef-building polypifers can exist only at limited depths. It follows
from this, that where the sea is very shallow, as in the Persian Gulf
and in parts of the East Indian Archipelago, the reefs lose their
fringing character, and appear as separate and irregularly scattered
patches, often of considerable area. From the more vigorous growth of
the coral on the outside, and from the conditions being less favourable
in several respects within, such reefs are generally higher and more
perfect in their marginal than in their central parts; hence these
reefs sometimes assume (and this circumstance ought not to be
overlooked) the appearance of atolls; but they differ from atolls in
their central expanse being much less deep, in their form being less
defined, and in being based on a shallow foundation. But when in a deep
sea reefs fringe banks of sediment, which have accumulated beneath the
surface, round either islands or submerged rocks, they are
distinguished with difficulty on the one hand from encircling
barrier-reefs, and on the other from atolls. In the West Indies there
are reefs, which I should probably have arranged under both these
classes, had not the existence of large and level banks, lying a little
beneath the surface, ready to serve as the basis for the attachment of
coral, been occasionally brought into view by the entire or partial
absence of reefs on them, and had not the formation of such banks,
through the accumulation of sediment now in progress, been sufficiently
evident. Fringing-reefs sometimes coat, and thus protect the
foundations of islands, which have been worn down by the surf to the
level of the sea. According to Ehrenberg, this has been extensively the
case with the islands in the Red Sea, which formerly ranged parallel to
the shores of the mainland, with deep water within them: hence the
reefs now coating their bases are situated relatively to the land like
barrier-reefs, although not belonging to that class; but there are, as
I believe, in the Red Sea some true barrier-reefs. The reefs of this
sea and of the West Indies will be described in the Appendix. In some
cases, fringing-reefs appear to be considerably modified in outline by
the course of the prevailing currents. Dr. J. Allan informs me that on
the east coast of Madagascar almost every headland and low point of
sand has a coral-reef extending from it in a S.W. and N.E. line,
parallel to the currents on that shore. I should think the influence of
the currents chiefly consisted in causing an extension, in a certain
direction, of a proper foundation for the attachment of the coral.
Round many intertropical islands, for instance the Abrolhos on the
coast of Brazil surveyed by Captain Fitzroy, and, as I am informed by
Mr. Cuming, round the Philippines, the bottom of the sea is entirely
coated by irregular masses of coral, which although often of large
size, do not reach the surface and form proper reefs. This must be
owing, either to insufficient growth, or to the absence of those kinds
of corals which can withstand the breaking of the waves.
The three classes, atoll-formed, barrier, and fringing-reefs, together
with the modifications just described of the latter, include all the
most remarkable coral formations anywhere existing. At the commencement
of the last chapter in the volume, where I detail the principles on
which the map (Plate III.) is coloured, the exceptional cases will be
enumerated.
CHAPTER IV.
ON THE DISTRIBUTION AND GROWTH OF CORAL-REEFS.
In this chapter I will give all the facts which I have collected,
relating to the distribution of coral-reefs,—to the conditions
favourable to their increase,—to the rate of their growth,—and to the
depth at which they are formed.
These subjects have an important bearing on the theory of the origin of
the different classes of coral-reefs.
SECTION I.—ON THE DISTRIBUTION OF CORAL-REEFS, AND ON THE CONDITIONS
FAVOURABLE TO THEIR INCREASE.
With regard to the limits of latitude, over which coral-reefs extend, I
have nothing new to add. The Bermuda Islands, in 32° 15′ N., is the
point furthest removed from the equator, in which they appear to exist;
and it has been suggested that their extension so far northward in this
instance is owing to the warmth of the Gulf Stream. In the Pacific, the
Loo Choo Islands, in latitude 27° N., have reefs on their shores, and
there is an atoll in 28° 30′, situated N.W. of the Sandwich
Archipelago. In the Red Sea there are coral-reefs in latitude 30°. In
the southern hemisphere coral-reefs do not extend so far from the
equatorial sea. In the Southern Pacific there are only a few reefs
beyond the line of the tropics, but Houtmans Abrolhos, on the western
shores of Australia in latitude 29° S., are of coral formation.
The proximity of volcanic land, owing to the lime generally evolved
from it, has been thought to be favourable to the increase of
coral-reefs. There is, however, not much foundation for this view; for
nowhere are coral-reefs more extensive than on the shores of New
Caledonia, and of north-eastern Australia, which consist of primary
formations; and in the largest groups of atolls, namely the Maldiva,
Chagos, Marshall, Gilbert, and Low Archipelagoes, there is no volcanic
or other kind of rock, excepting that formed of coral.
The entire absence of coral-reefs in certain large areas within the
tropical seas, is a remarkable fact. Thus no coral-reefs were observed,
during the surveying voyages of the “Beagle” and her tender on the west
coast of South America south of the equator, or round the Galapagos
Islands. It appears, also, that there are none[1] north of the equator;
Mr. Lloyd, who surveyed the Isthmus of Panama, remarked to me, that
although he had seen corals living in the Bay of Panama, yet he had
never observed any reefs formed by them. I at first attributed this
absence of reefs on the coasts of Peru and of the Galapagos Islands,[2]
to the coldness of the currents from the south, but the Gulf of Panama
is one of the hottest pelagic districts in the world.[3] In the central
parts of the Pacific there are islands entirely free from reefs; in
some few of these cases I have thought that this was owing to recent
volcanic action; but the existence of reefs round the greater part of
Hawaii, one of the Sandwich Islands, shows that recent volcanic action
does not necessarily prevent their growth.
[1] I have been informed that this is the case, by Lieutenant Ryder,
R.N., and others who have had ample opportunities for observation.
[2] The mean temperature of the surface sea from observations made by
the direction of Captain Fitzroy on the shores of the Galapagos
Islands, between the 16th of September and the 20th of October, 1835,
was 68° Fahr. The lowest temperature observed was 58.5° at the
south-west end of Albemarle Island; and on the west coast of this
island, it was several times 62° and 63°. The mean temperature of the
sea in the Low Archipelago of atolls, and near Tahiti, from similar
observations made on board the _Beagle_, was (although further from
the equator) 77.5°, the lowest any day being 76.5°. Therefore we have
here a difference of 9.5° in mean temperature, and 18° in extremes; a
difference doubtless quite sufficient to affect the distribution of
organic beings in the two areas.
[3] Humboldt’s “Personal Narrative,” volume vii., page 434.
In the last chapter I stated that the bottom of the sea round some
islands is thickly coated with living corals, which nevertheless do not
form reefs, either from insufficient growth, or from the species not
being adapted to contend with the breaking waves.
I have been assured by several people, that there are no coral-reefs on
the west coast of Africa,[4] or round the islands in the Gulf of
Guinea. This perhaps may be attributed, in part, to the sediment
brought down by the many rivers debouching on that coast, and to the
extensive mud-banks, which line great part of it. But the islands of
St. Helena, Ascension, the Cape Verdes, St. Paul’s, and Fernando
Noronha, are, also, entirely without reefs, although they lie far out
at sea, are composed of the same ancient volcanic rocks, and have the
same general form, with those islands in the Pacific, the shores of
which are surrounded by gigantic walls of coral-rock. With the
exception of Bermuda, there is not a single coral-reef in the central
expanse of the Atlantic Ocean. It will, perhaps, be suggested that the
quantity of carbonate of lime in different parts of the sea, may
regulate the presence of reefs. But this cannot be the case, for at
Ascension, the waves charged to excess precipitate a thick layer of
calcareous matter on the tidal rocks; and at St. Jago, in the Cape
Verdes, carbonate of lime not only is abundant on the shores, but it
forms the chief part of some upraised post-tertiary strata. The
apparently capricious distribution, therefore, of coral-reefs, cannot
be explained by any of these obvious causes; but as the study of the
terrestrial and better known half of the world must convince every one
that no station capable of supporting life is lost,—nay more, that
there is a struggle for each station, between the different orders of
nature,—we may conclude that in those parts of the intertropical sea,
in which there are no coral-reefs, there are other organic bodies
supplying the place of the reef-building polypifers. It has been shown
in the chapter on Keeling atoll that there are some species of large
fish, and the whole tribe of Holothuriæ which prey on the tenderer
parts of the corals. On the other hand, the polypifers in their turn
must prey on some other organic beings; the decrease of which from any
cause would cause a proportionate destruction of the living coral. The
relations, therefore, which determine the formation of reefs on any
shore, by the vigorous growth of the efficient kinds of coral, must be
very complex, and with our imperfect knowledge quite inexplicable. From
these considerations, we may infer that changes in the condition of the
sea, not obvious to our senses, might destroy all the coral-reefs in
one area, and cause them to appear in another: thus, the Pacific or
Indian Ocean might become as barren of coral-reefs as the Atlantic now
is, without our being able to assign any adequate cause for such a
change.
[4] It might be concluded, from a paper by Captain Owen (“Geographical
Journal”, volume ii., page 89), that the reefs off Cape St. Anne and
the Sherboro’ Islands were of coral, although the author states that
they are not purely coralline. But I have been assured by Lieutenant
Holland, R.N., that these reefs are not of coral, or at least that
they do not at all resemble those in the West Indies.
It has been a question with some naturalists, which part of a reef is
most favourable to the growth of coral. The great mounds of living
Porites and of Millepora round Keeling atoll occur exclusively on the
extreme verge of the reef, which is washed by a constant succession of
breakers; and living coral nowhere else forms solid masses. At the
Marshall islands the larger kinds of coral (chiefly species of Astræa,
a genus closely allied to Porites) “which form rocks measuring several
fathoms in thickness,” prefer, according to Chamisso,[5] the most
violent surf. I have stated that the outer margin of the Maldiva atolls
consists of living corals (some of which, if not all, are of the same
species with those at Keeling atoll), and here the surf is so
tremendous, that even large ships have been thrown, by a single heave
of the sea, high and dry on the reef, all on board thus escaping with
their lives.
[5] Kotzebue’s “First Voyage” (English Translation) volume iii., pages
142, 143, 331.
Ehrenberg[6] remarks, that in the Red Sea the strongest corals live on
the outer reefs, and appear to love the surf; he adds, that the more
branched kinds abound a little way within, but that even these in still
more protected places, become smaller. Many other facts having a
similar tendency might be adduced.[7] It has, however, been doubted by
MM. Quoy and Gaimard, whether any kind of coral can even withstand,
much less flourish in, the breakers of an open sea:[8] they affirm that
the saxigenous lithophytes flourish only where the water is tranquil,
and the heat intense. This statement has passed from one geological
work to another; nevertheless, the protection of the whole reef
undoubtedly is due to those kinds of coral, which cannot exist in the
situations thought by these naturalists to be most favourable to them.
For should the outer and living margin perish, of any one of the many
low coral-islands, round which a line of great breakers is incessantly
foaming, the whole, it is scarcely possible to doubt, would be washed
away and destroyed, in less than half a century. But the vital energies
of the corals conquer the mechanical power of the waves; and the large
fragments of reef torn up by every storm, are replaced by the slow but
steady growth of the innumerable polypifers, which form the living zone
on its outer edge.
[6] Ehrenberg, “Über die Natür und Bildung der Corallen Bänke im
rothen Meere,” page 49.
[7] In the West Indies, as I am informed by Captain Bird Allen, R.N.,
it is the common belief of those, who are best acquainted with the
reefs, that the coral flourishes most, where freely exposed to the
swell of the open sea.
[8] “Annales des Sciences Naturelles,” tome vi., pages 276, 278.—“Là
où les ondes sont agitées, les Lytophytés ne peuvent travailler, parce
qu’elles détruiraient leurs fragiles édifices,” &c.
From these facts, it is certain, that the strongest and most massive
corals flourish, where most exposed. The less perfect state of the reef
of most atolls on the leeward and less exposed side, compared with its
state to windward; and the analogous case of the greater number of
breaches on the near sides of those atolls in the Maldiva Archipelago,
which afford some protection to each other, are obviously explained by
this circumstance. If the question had been, under what conditions the
greater number of species of coral, not regarding their bulk and
strength, were developed, I should answer,—probably in the situations
described by MM. Quoy and Gaimard, where the water is tranquil and the
heat intense. The total number of species of coral in the
circumtropical seas must be very great: in the Red Sea alone, 120
kinds, according to Ehrenberg,[9] have been observed.
[9] Ehrenberg, “Über die Natür,” etc., etc., page 46.
The same author has observed that the recoil of the sea from a steep
shore is injurious to the growth of coral, although waves breaking over
a bank are not so. Ehrenberg also states, that where there is much
sediment, placed so as to be liable to be moved by the waves there is
little or no coral; and a collection of living specimens placed by him
on a sandy shore died in the course of a few days.[10] An experiment,
however, will presently be related in which some large masses of living
coral increased rapidly in size, after having been secured by stakes on
a sandbank. That loose sediment should be injurious to the living
polypifers, appears, at first sight, probable; and accordingly, in
sounding off Keeling atoll, and (as will hereafter be shown) off
Mauritius, the arming of the lead invariably came up clean, where the
coral was growing vigorously. This same circumstance has probably given
rise to a strange belief, which, according to Captain Owen,[11] is
general amongst the inhabitants of the Maldiva atolls, namely that
corals have roots, and therefore that if merely broken down to the
surface, they grow up again; but, if rooted out, they are permanently
destroyed. By this means the inhabitants keep their harbours clear; and
thus the French Governor of St. Mary’s in Madagascar, “cleared out and
made a beautiful little port at that place.” For it is probable that
sand would accumulate in the hollows formed by tearing out the corals,
but not on the broken and projecting stumps, and therefore, in the
former case, the fresh growth of the coral might be thus prevented.
[10] Ibid., page 49.
[11] Captain Owen on the Geography of the Maldiva Islands,
“Geographical Journal”, volume ii., page 88.
In the last chapter I remarked that fringing-reefs are almost
universally breached, where streams enter the sea.[12] Most authors
have attributed this fact to the injurious effects of the fresh water,
even where it enters the sea only in small quantity, and during a part
of the year. No doubt brackish water would prevent or retard the growth
of coral; but I believe that the mud and sand which is deposited, even
by rivulets when flooded, is a much more efficient check. The reef on
each side of the channel leading into Port Louis at Mauritius, ends
abruptly in a wall, at the foot of which I sounded and found a bed of
thick mud. This steepness of the sides appears to be a general
character in such breaches. Cook,[13] speaking of one at Raiatea, says,
“like all the rest, it is very steep on both sides.” Now, if it were
the fresh water mingling with the salt which prevented the growth of
coral, the reef certainly would not terminate abruptly, but as the
polypifers nearest the impure stream would grow less vigorously than
those farther off, so would the reef gradually thin away. On the other
hand, the sediment brought down from the land would only prevent the
growth of the coral in the line of its deposition, but would not check
it on the side, so that the reefs might increase till they overhung the
bed of the channel. The breaches are much fewer in number, and front
only the larger valleys in reefs of the encircling barrier class. They
probably are kept open in the same manner as those into the lagoon of
an atoll, namely, by the force of the currents and the drifting
outwards of fine sediment. Their position in front of valleys, although
often separated from the land by deep water lagoon-channels, which it
might be thought would entirely remove the injurious effects both of
the fresh water and the sediment, will receive a simple explanation
when we discuss the origin of barrier-reefs.
[12] Lieutenant Wellstead and others have remarked that this is the
case in the Red Sea; Dr. Rüppell (“Reise in Abyss.” Band. i., page
142) says that there are pear-shaped harbours in the upraised
coral-coast, into which periodical streams enter. From this
circumstance, I presume, we must infer that before the upheaval of the
strata now forming the coast-land, fresh water and sediment entered
the sea at these points; and the coral being thus prevented growing,
the pear-shaped harbours were produced.
[13] Cook’s “First Voyage,” volume ii., page 271 (Hawkesworth’s
edition).)
In the vegetable kingdom every different station has its peculiar group
of plants, and similar relations appear to prevail with corals. We have
already described the great difference between the corals within the
lagoon of an atoll and those on its outer margin. The corals, also, on
the margin of Keeling Island occurred in zones; thus the _Porites_ and
_Millepora complanata_ grow to a large size only where they are washed
by a heavy sea, and are killed by a short exposure to the air; whereas,
three species of Nullipora also live amidst the breakers, but are able
to survive uncovered for a part of each tide; at greater depths, a
strong Madrepora and _Millepora alcicornis_ are the commonest kinds,
the former appearing to be confined to this part, beneath the zone of
massive corals, minute encrusting corallines and other organic bodies
live. If we compare the external margin of the reef at Keeling atoll
with that on the leeward side of Mauritius, which are very differently
circumstanced, we shall find a corresponding difference in the
appearance of the corals. At the latter place, the genus Madrepora is
preponderant over every other kind, and beneath the zone of massive
corals there are large beds of Seriatopora. There is also a marked
difference, according to Captain Moresby,[14] between the great
branching corals of the Red Sea, and those on the reefs of the Maldiva
atolls.
[14] Captain Moresby on the Northern Maldiva atolls, “Geographical
Journal”, volume v., page 401.
These facts, which in themselves are deserving of notice, bear,
perhaps, not very remotely, on a remarkable circumstance which has been
pointed out to me by Captain Moresby, namely, that with very few
exceptions, none of the coral-knolls within the lagoons of Peros
Banhos, Diego Garcia, and the Great Chagos Bank (all situated in the
Chagos group), rise to the surface of the water; whereas all those,
with equally few exceptions, within Solomon and Egmont atolls in the
same group, and likewise within the large southern Maldiva atolls,
reach the surface. I make these statements, after having examined the
charts of each atoll. In the lagoon of Peros Banhos, which is nearly
twenty miles across, there is only one single reef which rises to the
surface; in Diego Garcia there are seven, but several of these lie
close to the margin of the lagoon, and need scarcely have been
reckoned; in the Great Chagos Bank there is not one. On the other hand,
in the lagoons of some of the great southern Maldiva atolls, although
thickly studded with reefs, every one without exception rises to the
surface; and on an average there are less than two submerged reefs in
each atoll; in the northern atolls, however, the submerged lagoon-reefs
are not quite so rare. The submerged reefs in the Chagos atolls
generally have from one to seven fathoms water on them, but some have
from seven to ten. Most of them are small with very steep sides;[15] at
Peros Banhos they rise from a depth of about thirty fathoms, and some
of them in the Great Chagos Bank from above forty fathoms; they are
covered, Captain Moresby informs me, with living and healthy coral, two
and three feet high, consisting of several species. Why then have not
these lagoon-reefs reached the surface, like the innumerable ones in
the atolls above named? If we attempt to assign any difference in their
external conditions, as the cause of this diversity, we are at once
baffled. The lagoon of Diego Garcia is not deep, and is almost wholly
surrounded by its reef; Peros Banhos is very deep, much larger, with
many wide passages communicating with the open sea. On the other hand,
of those atolls, in which all or nearly all the lagoon-reefs have
reached the surface, some are small, others large, some shallow, others
deep, some well-enclosed, and others open.
[15] Some of these statements were not communicated to me verbally by
Captain Moresby, but are taken from the MS. account before alluded to,
of the Chagos Group.
Captain Moresby informs me that he has seen a French chart of Diego
Garcia made eighty years before his survey, and apparently very
accurate; and from it he infers, that during this interval there has
not been the smallest change in the depth on any of the knolls within
the lagoon. It is also known that during the last fifty-one years, the
eastern channel into the lagoon has neither become narrower, nor
decreased in depth; and as there are numerous small knolls of living
coral within it, some change might have been anticipated. Moreover, as
the whole reef round the lagoon of this atoll has been converted into
land—an unparalleled case, I believe, in an atoll of such large
size,—and as the strip of land is for considerable spaces more than
half a mile wide—also a very unusual circumstance,—we have the best
possible evidence, that Diego Garcia has remained at its present level
for a very long period. With this fact, and with the knowledge that no
sensible change has taken place during eighty years in the
coral-knolls, and considering that every single reef has reached the
surface in other atolls, which do not present the smallest appearance
of being older than Diego Garcia and Peros Banhos, and which are placed
under the same external conditions with them, one is led to conclude
that these submerged reefs, although covered with luxuriant coral, have
no tendency to grow upwards, and that they would remain at their
present levels for an almost indefinite period.
From the number of these knolls, from their position, size, and form,
many of them being only one or two hundred yards across, with a rounded
outline, and precipitous sides,—it is indisputable that they have been
formed by the growth of coral; and this makes the case much more
remarkable. In Peros Banhos and in the Great Chagos Bank, some of these
almost columnar masses are 200 feet high, and their summits lie only
from two to eight fathoms beneath the surface; therefore, a small
proportional amount more of growth would cause them to attain the
surface, like those numerous knolls, which rise from an equally great
depth within the Maldiva atolls. We can hardly suppose that time has
been wanting for the upward growth of the coral, whilst in Diego
Garcia, the broad annular strip of land, formed by the continued
accumulation of detritus, shows how long this atoll has remained at its
present level. We must look to some other cause than the rate of
growth; and I suspect it will be found in the reefs being formed of
different species of corals, adapted to live at different depths.
The Great Chagos Bank is situated in the centre of the Chagos Group,
and the Pitt and Speaker Banks at its two extreme points. These banks
resemble atolls, except in their external rim being about eight fathoms
submerged, and in being formed of dead rock, with very little living
coral on it: a portion nine miles long of the annular reef of Peros
Banhos atoll is in the same condition. These facts, as will hereafter
be shown, render it very probable that the whole group at some former
period subsided seven or eight fathoms; and that the coral perished on
the outer margin of those atolls which are now submerged, but that it
continued alive, and grew up to the surface on those which are now
perfect. If these atolls did subside, and if from the suddenness of the
movement or from any other cause, those corals which are better adapted
to live at a certain depth than at the surface, once got possession of
the knolls, supplanting the former occupants, they would exert little
or no tendency to grow upwards. To illustrate this, I may observe, that
if the corals of the upper zone on the outer edge of Keeling atoll were
to perish, it is improbable that those of the lower zone would grow to
the surface, and thus become exposed to conditions for which they do
not appear to be adapted. The conjecture, that the corals on the
submerged knolls within the Chagos atolls have analogous habits with
those of the lower zone outside Keeling atoll, receives some support
from a remark by Captain Moresby, namely, that they have a different
appearance from those on the reefs in the Maldiva atolls, which, as we
have seen, all rise to the surface: he compares the kind of difference
to that of the vegetation under different climates. I have entered at
considerable length into this case, although unable to throw much light
on it, in order to show that an equal tendency to upward growth ought
not to be attributed to all coral-reefs,—to those situated at different
depths,—to those forming the ring of an atoll or those on the knolls
within a lagoon,—to those in one area and those in another. The
inference, therefore, that one reef could not grow up to the surface
within a given time, because another, not known to be covered with the
same species of corals, and not known to be placed under conditions
exactly the same, has not within the same time reached the surface, is
unsound.
SECTION II.—ON THE RATE OF GROWTH OF CORAL-REEFS.
The remark made at the close of the last section, naturally leads to
this division of our subject, which has not, I think, hitherto been
considered under a right point of view. Ehrenberg[16] has stated, that
in the Red Sea, the corals only coat other rocks in a layer from one to
two feet in thickness, or at most to a fathom and a half; and he
disbelieves that, in any case, they form, by their own proper growth,
great masses, stratum over stratum. A nearly similar observation has
been made by MM. Quoy and Gaimard,[17] with respect to the thickness of
some upraised beds of coral, which they examined at Timor and some
other places. Ehrenberg[18] saw certain large massive corals in the Red
Sea, which he imagines to be of such vast antiquity, that they might
have been beheld by Pharaoh; and according to Mr. Lyell[19] there are
certain corals at Bermuda, which are known by tradition, to have been
living for centuries. To show how slowly coral-reefs grow upwards,
Captain Beechey[20] has adduced the case of the Dolphin Reef off
Tahiti, which has remained at the same depth beneath the surface,
namely about two fathoms and a half, for a period of sixty-seven years.
There are reefs in the Red Sea, which certainly do not appear[21] to
have increased in dimensions during the last half-century, and from the
comparison of old charts with recent surveys, probably not during the
last two hundred years. These, and other similar facts, have so
strongly impressed many with the belief of the extreme slowness of the
growth of corals, that they have even doubted the possibility of
islands in the great oceans having been formed by their agency. Others,
again, who have not been overwhelmed by this difficulty, have admitted
that it would require thousands, and tens of thousands of years, to
form a mass, even of inconsiderable thickness; but the subject has not,
I believe, been viewed in the proper light.
[16] Ehrenberg, as before cited, pages 39, 46, and 50.
[17] “Annales des Sciences Nat.” tom. vi., page 28.
[18] Ehrenberg, ut sup., page 42.
[19] Lyell’s “Principles of Geology,” book iii., chapter xviii.
[20] Beechey’s “Voyage to the Pacific,” chapter viii.
[21] Ehrenberg, ut sup., page 43.
That masses of considerable thickness have been formed by the growth of
coral, may be inferred with certainty from the following facts. In the
deep lagoons of Peros Banhos and of the Great Chagos Bank, there are,
as already described, small steep-sided knolls covered with living
coral. There are similar knolls in the southern Maldiva atolls, some of
which, as Captain Moresby assures me, are less than a hundred yards in
diameter, and rise to the surface from a depth of between two hundred
and fifty and three hundred feet. Considering their number, form, and
position, it would be preposterous to suppose that they are based on
pinnacles of any rock, not of coral formation; or that sediment could
have been heaped up into such small and steep isolated cones. As no
kind of living coral grows above the height of a few feet, we are
compelled to suppose that these knolls have been formed by the
successive growth and death of many individuals,—first one being broken
off or killed by some accident, and then another, and one set of
species being replaced by another set with different habits, as the
reef rose nearer the surface, or as other changes supervened. The
spaces between the corals would become filled up with fragments and
sand, and such matter would probably soon be consolidated, for we learn
from Lieutenant Nelson,[22] that at Bermuda a process of this kind
takes place beneath water, without the aid of evaporation. In reefs,
also, of the barrier class, we may feel sure, as I have shown, that
masses of great thickness have been formed by the growth of the coral;
in the case of Vanikoro, judging only from the depth of the moat
between the land and the reef, the wall of coral-rock must be at least
three hundred feet in vertical thickness.
[22] “Geological Transactions,” volume v., page 113.
It is unfortunate that the upraised coral-islands in the Pacific have
not been examined by a geologist. The cliffs of Elizabeth Island, in
the Low Archipelago, are eighty feet high, and appear, from Captain
Beechey’s description, to consist of a homogeneous coral-rock. From the
isolated position of this island, we may safely infer that it is an
upraised atoll, and therefore that it has been formed by masses of
coral, grown together. Savage Island seems, from the description of the
younger Forster,[23] to have a similar structure, and its shores are
about forty feet high: some of the Cook Islands also appear[24] to be
similarly composed. Captain Belcher, R.N., in a letter which Captain
Beaufort showed me at the admiralty, speaking of Bow atoll, says, “I
have succeeded in boring forty-five feet through coral-sand, when the
auger became jammed by the falling in of the surrounding _creamy_
matter.” On one of the Maldiva atolls, Captain Moresby bored to a depth
of twenty-six feet, when his auger also broke: he has had the kindness
to give me the matter brought up; it is perfectly white, and like
finely triturated coral-rock.
[23] Forster’s “Voyage round the World with Cook,” volume ii., pages
163, 167.
[24] Williams’s “Narrative of Missionary Enterprise,” page 30.
In my description of Keeling atoll, I have given some facts, which show
that the reef probably has grown outwards; and I have found, just
within the outer margin, the great mounds of Porites and of Millepora,
with their summits lately killed, and their sides subsequently
thickened by the growth of the coral: a layer, also, of Nullipora had
already coated the dead surface. As the external slope of the reef is
the same round the whole of this atoll, and round many other atolls,
the angle of inclination must result from an adaption between the
growing powers of the coral, and the force of the breakers, and their
action on the loose sediment. The reef, therefore, could not increase
outwards, without a nearly equal addition to every part of the slope,
so that the original inclination might be preserved, and this would
require a large amount of sediment, all derived from the wear of corals
and shells, to be added to the lower part. Moreover, at Keeling atoll,
and probably in many other cases, the different kinds of corals would
have to encroach on each other; thus the Nulliporæ cannot increase
outwards without encroaching on the Porites and _Millepora complanata_,
as is now taking place; nor these latter without encroaching on the
strongly branched Madreporet, the _Millepora alcicornis_, and some
Astræas; nor these again without a foundation being formed for them
within the requisite depth, by the accumulation of sediment. How slow,
then, must be the ordinary lateral or outward growth of such reefs. But
off Christmas atoll, where the sea is much more shallow than is usual,
we have good reason to believe that, within a period not very remote,
the reef has increased considerably in width. The land has the
extraordinary breadth of three miles; it consists of parallel ridges of
shells and broken corals, which furnish “an incontestable proof,” as
observed by Cook,[25] “that the island has been produced by accessions
from the sea, and is in a state of increase.” The land is fronted by a
coral-reef, and from the manner in which islets are known to be formed,
we may feel confident that the reef was not three miles wide, when the
first, or most backward ridge, was thrown up; and, therefore, we must
conclude that the reef has grown outwards during the accumulation of
the successive ridges. Here then, a wall of coral-rock of very
considerable breadth has been formed by the outward growth of the
living margin, within a period during which ridges of shells and
corals, lying on the bare surface, have not decayed. There can be
little doubt, from the account given by Captain Beechey, that Matilda
atoll, in the Low Archipelago, has been converted in the space of
thirty-four years, from being, as described by the crew of a wrecked
whaling vessel, a “reef of rocks” into a lagoon-island, fourteen miles
in length, with “one of its sides covered nearly the whole way with
high trees.”[26] The islets, also, on Keeling atoll, it has been shown,
have increased in length, and since the construction of an old chart,
several of them have become united into one long islet; but in this
case, and in that of Matilda atoll, we have no proof, and can only
infer as probable, that the reef, that is the foundation of the islets,
has increased as well as the islets themselves.
[25] Cook’s “Third Voyage,” book III., chapter x.
[26] Beechey’s “Voyage to the Pacific,” chapter vii. and viii.
After these considerations, I attach little importance, as indicating
the ordinary and still less the possible rate of _outward_ growth of
coral-reefs, to the fact that certain reefs in the Red Sea have not
increased during a long interval of time; or to other such cases, as
that of Ouluthy atoll in the Caroline group, where every islet,
described a thousand years before by Cantova was found in the same
state by Lutké,[27]—without it could be shown that, in these cases, the
conditions were favourable to the vigorous and unopposed growth of the
corals living in the different zones of depth, and that a proper basis
for the extent of the reef was present. The former conditions must
depend on many contingencies, and in the deep oceans where coral
formations most abound, a basis within the requisite depth can rarely
be present.
[27] F. Lutké’s “Voyage autour du Monde.” In the group Elato, however,
it appears that what is now the islet Falipi, is called in Cantova’s
Chart, the Banc de Falipi. It is not stated whether this has been
caused by the growth of coral, or by the accumulation of sand.
Nor do I attach any importance to the fact of certain submerged reefs,
as those off Tahiti, or those within Diego Garcia not now being nearer
the surface than they were many years ago, as an indication of the rate
under favourable circumstances of the _upward_ growth of reefs; after
it has been shown, that all the reefs have grown to the surface in some
of the Chagos atolls, but that in neighbouring atolls which appear to
be of equal antiquity and to be exposed to the same external
conditions, every reef remains submerged; for we are almost driven to
attribute this to a difference, not in the rate of growth, but in the
habits of the corals in the two cases.
In an old-standing reef, the corals, which are so different in kind on
different parts of it, are probably all adapted to the stations they
occupy, and hold their places, like other organic beings, by a struggle
one with another, and with external nature; hence we may infer that
their growth would generally be slow, except under peculiarly
favourable circumstances. Almost the only natural condition, allowing a
quick upward growth of the whole surface of a reef, would be a slow
subsidence of the area in which it stood; if, for instance, Keeling
atoll were to subside two or three feet, can we doubt that the
projecting margin of live coral, about half an inch in thickness, which
surrounds the dead upper surfaces of the mounds of Porites, would in
this case form a concentric layer over them, and the reef thus increase
upwards, instead of, as at present, outwards? The Nulliporæ are now
encroaching on the Porites and Millepora, but in this case might we not
confidently expect that the latter would, in their turn, encroach on
the Nulliporæ? After a subsidence of this kind, the sea would gain on
the islets, and the great fields of dead but upright corals in the
lagoon, would be covered by a sheet of clear water; and might we not
then expect that these reefs would rise to the surface, as they
anciently did when the lagoon was less confined by islets, and as they
did within a period of ten years in the schooner-channel, cut by the
inhabitants? In one of the Maldiva atolls, a reef, which within a very
few years existed as an islet bearing cocoa-nut trees, was found by
Lieutenant Prentice “_entirely covered with live coral and Madrepore_.”
The natives believe that the islet was washed away by a change in the
currents, but if, instead of this, it had quietly subsided, surely
every part of the island which offered a solid foundation, would in a
like manner have become coated with living coral.
Through steps such as these, any thickness of rock, composed of a
singular intermixture of various kinds of corals, shells, and
calcareous sediment, might be formed; but without subsidence, the
thickness would necessarily be determined by the depth at which the
reef-building polypifers can exist. If it be asked, at what rate in
years I suppose a reef of coral favourably circumstanced could grow up
from a given depth; I should answer, that we have no precise evidence
on this point, and comparatively little concern with it. We see, in
innumerable points over wide areas, that the rate has been sufficient,
either to bring up the reefs from various depths to the surface, or, as
is more probable, to keep them at the surface, during progressive
subsidences; and this is a much more important standard of comparison
than any cycle of years.
It may, however, be inferred from the following facts, that the rate in
years under favourable circumstances would be very far from slow. Dr.
Allan, of Forres, has, in his MS. Thesis deposited in the library of
the Edinburgh University (extracts from which I owe to the kindness of
Dr. Malcolmson), the following account of some experiments, which he
tried during his travels in the years 1830 to 1832 on the east coast of
Madagascar. “To ascertain the rise and progress of the coral-family,
and fix the number of species met with at Foul Point (latitude 17° 40′)
twenty species of coral were taken off the reef and planted apart on a
sand-bank _three feet deep at low water_. Each portion weighed ten
pounds, and was kept in its place by stakes. Similar quantities were
placed in a clump and secured as the rest. This was done in December
1830. In July following, each detached mass was nearly level with the
sea at low water, quite immovable, and several feet long, stretching as
the parent reef, with the coast current from north to south. The masses
accumulated in a clump were found equally increased, but some of the
species in such unequal ratios, as to be growing over each other.” The
loss of Dr. Allan’s magnificent collection by shipwreck, unfortunately
prevents its being known to what genera these corals belonged; but from
the numbers experimented on, it is certain that all the more
conspicuous kinds must have been included. Dr. Allan informs me, in a
letter, that he believes it was a Madrepora, which grew most
vigorously. One may be permitted to suspect that the level of the sea
might possibly have been somewhat different at the two stated periods;
nevertheless, it is quite evident that the growth of the ten-pound
masses, during the six or seven months, at the end of which they were
found immovably fixed[28] and several feet in length, must have been
very great. The fact of the different kinds of coral, when placed in
one clump, having increased in extremely unequal ratios, is very
interesting, as it shows the manner in which a reef, supporting many
species of coral, would probably be affected by a change in the
external conditions favouring one kind more than another. The growth of
the masses of coral in N. and S. lines parallel to the prevailing
currents, whether due to the drifting of sediment or to the simple
movement of the water, is, also, a very interesting circumstance.
[28] It is stated by De la Beche (“Geological Manual,” page 143), on
the authority of Mr. Lloyd, who surveyed the Isthmus of Panama, that
some specimens of Polypifers, placed by him in a sheltered pool of
water, were found in the course of a few days firmly fixed by the
secretion of a stony matter, to the bottom.
A fact, communicated to me by Lieutenant Wellstead, I.N., in some
degree corroborates the result of Dr. Allan’s experiments: it is, that
in the Persian Gulf a ship had her copper bottom encrusted in the
course of twenty months with a layer of coral, _two feet_ in thickness,
which it required great force to remove, when the vessel was docked: it
was not ascertained to what order this coral belonged. The case of the
schooner-channel choked up with coral in an interval of less than ten
years, in the lagoon of Keeling atoll, should be here borne in mind. We
may also infer, from the trouble which the inhabitants of the Maldiva
atolls take to root out, as they express it, the coral-knolls from
their harbours, that their growth can hardly be very slow.[29]
[29] Mr. Stutchbury (“West of England Journal”, No. I., page 50.) has
described a specimen of Agaricia, “weighing 2 lbs. 9 oz., which
surrounds a species of oyster, whose age could not be more than two
years, and yet is completely enveloped by this dense coral.” I presume
that the oyster was living when the specimen was procured; otherwise
the fact tells nothing. Mr. Stutchbury also mentions an anchor, which
had become entirely encrusted with coral in fifty years; other cases,
however, are recorded of anchors which have long remained amidst
coral-reefs without having become coated. The anchor of the _Beagle_,
in 1832, after having been down exactly one month at Rio de Janeiro,
was so thickly coated by two species of Tubularia, that large spaces
of the iron were entirely concealed; the tufts of this horny zoophyte
were between two and three inches in length. It has been attempted to
compute, but I believe erroneously, the rate of growth of a reef, from
the fact mentioned by Captain Beechey, of the _Chama gigas_ being
embedded in coral-rock. But it should be remembered, that some species
of this genus invariably live, both whilst young and old, in cavities,
which the animal has the power of enlarging with its growth. I saw
many of these shells thus embedded in the outer “flat” of Keeling
atoll, which is composed of dead rock; and therefore the cavities in
this case had no relation whatever with the growth of coral. M.
Lesson, also, speaking of this shell (Partie Zoolog. “Voyage de la
‘Coquille’”), has remarked, “que constamment ses valves étaient
engagés complètement dans la masse des Madrepores.”
From the facts given in this section, it may be concluded, first, that
considerable thicknesses of rock have certainly been formed within the
present geological area by the growth of coral and the accumulation of
its detritus; and, secondly, that the increase of individual corals and
of reefs, both outwards or horizontally and upwards or vertically,
under the peculiar conditions favourable to such increase, is not slow,
when referred either to the standard of the average oscillations of
level in the earth’s crust, or to the more precise but less important
one of a cycle of years.
SECTION III.—ON THE DEPTHS AT WHICH REEF-BUILDING POLYPIFERS CAN LIVE.
I have already described in detail, which might have appeared trivial,
the nature of the bottom of the sea immediately surrounding Keeling
atoll; and I will now describe with almost equal care the soundings off
the fringing-reefs of Mauritius. I have preferred this arrangement, for
the sake of grouping together facts of a similar nature. I sounded with
the wide bell-shaped lead which Captain Fitzroy used at Keeling Island,
but my examination of the bottom was confined to a few miles of coast
(between Port Louis and Tomb Bay) on the leeward side of the island.
The edge of the reef is formed of great shapeless masses of branching
Madrepores, which chiefly consist of two species,—apparently _M.
corymbosa_ and _pocillifera_,— mingled with a few other kinds of coral.
These masses are separated from each other by the most irregular
gullies and cavities, into which the lead sinks many feet. Outside this
irregular border of Madrepores, the water deepens gradually to twenty
fathoms, which depth generally is found at the distance of from half to
three-quarters of a mile from the reef. A little further out the depth
is thirty fathoms, and thence the bank slopes rapidly into the depths
of the ocean. This inclination is very gentle compared with that
outside Keeling and other atolls, but compared with most coasts it is
steep. The water was so clear outside the reef, that I could
distinguish every object forming the rugged bottom. In this part, and
to a depth of eight fathoms, I sounded repeatedly, and at each cast
pounded the bottom with the broad lead, nevertheless the arming
invariably came up perfectly clean, but deeply indented. From eight to
fifteen fathoms a little calcareous sand was occasionally brought up,
but more frequently the arming was simply indented. In all this space
the two Madrepores above mentioned, and two species of Astræa, with
rather large[30] stars, seemed the commonest kinds; and it must be
noticed that twice at the depth of fifteen fathoms, the arming was
marked with a clean impression of an Astræa. Besides these lithophytes,
some fragments of the _Millepora alcicornis_, which occurs in the same
relative position at Keeling Island, were brought up; and in the deeper
parts there were large beds of a Seriatopora, different from _S.
subulata_, but closely allied to it. On the beach within the reef, the
rolled fragments consisted chiefly of the corals just mentioned, and of
a massive Porites, like that at Keeling atoll, of a Meandrina,
_Pocillopora verrucosa_, and of numerous fragments of Nullipora. From
fifteen to twenty fathoms the bottom was, with few exceptions, either
formed of sand, or thickly covered with Seriatopora: this delicate
coral seems to form at these depths extensive beds unmingled with any
other kind. At twenty fathoms, one sounding brought up a fragment of
Madrepora apparently _M. pocillifera_, and I believe it is the same
species (for I neglected to bring specimens from both stations) which
mainly forms the upper margin of the reef; if so, it grows in depths
varying from 0 to 20 fathoms. Between 20 and 23 fathoms I obtained
several soundings, and they all showed a sandy bottom, with one
exception at 30 fathoms, when the arming came up scooped out, as if by
the margin of a large Caryophyllia. Beyond 33 fathoms I sounded only
once; and from 86 fathoms, at the distance of one mile and a third from
the edge of the reef, the arming brought up calcareous sand with a
pebble of volcanic rock. The circumstance of the arming having
invariably come up quite clean, when sounding within a certain number
of fathoms off the reefs of Mauritius and Keeling atoll (eight fathoms
in the former case, and twelve in the latter) and of its having always
come up (with one exception) smoothed and covered with sand, when the
depth exceeded twenty fathoms, probably indicates a criterion, by which
the limits of the vigorous growth of coral might in all cases be
readily ascertained. I do not, however, suppose that if a vast number
of soundings were obtained round these islands, the limit above
assigned would be found never to vary, but I conceive the facts are
sufficient to show, that the exceptions would be few. The circumstance
of a _gradual_ change, in the two cases, from a field of clean coral to
a smooth sandy bottom, is far more important in indicating the depth at
which the larger kinds of coral flourish than almost any number of
separate observations on the depth, at which certain species have been
dredged up. For we can understand the gradation, only as a prolonged
struggle against unfavourable conditions. If a person were to find the
soil clothed with turf on the banks of a stream of water, but on going
to some distance on one side of it, he observed the blades of grass
growing thinner and thinner, with intervening patches of sand, until he
entered a desert of sand, he would safely conclude, especially if
changes of the same kind were noticed in other places, that the
presence of the water was absolutely necessary to the formation of a
thick bed of turf: so may we conclude, with the same feeling of
certainty, that thick beds of coral are formed only at small depths
beneath the surface of the sea.
[30] Since the preceding pages were printed off, I have received from
Mr. Lyell a very interesting pamphlet, entitled “Remarks upon Coral
Formations,” etc., by J. Couthouy, Boston, United States, 1842. There
is a statement (page 6), on the authority of the Rev. J. Williams,
corroborating the remarks made by Ehrenberg and Lyell (page 71 of this
volume), on the antiquity of certain individual corals in the Red Sea
and at Bermuda; namely, that at Upolu, one of the Navigator Islands,
“particular clumps of coral are known to the fishermen by name,
derived from either some particular configuration or tradition
attached to them, and handed down from time immemorial.” With respect
to the thickness of masses of coral-rock, it clearly appears, from the
descriptions given by Mr. Couthouy (pages 34, 58) that Mangaia and
Aurora Islands are upraised atolls, composed of coral rock: the level
summit of the former is about three hundred feet, and that of Aurora
Island is two hundred feet above the sea-level.
I have endeavoured to collect every fact, which might either invalidate
or corroborate this conclusion. Captain Moresby, whose opportunities
for observation during his survey of the Maldiva and Chagos
Archipelagoes have been unrivalled, informs me, that the upper part or
zone of the steep-sided reefs, on the inner and outer coasts of the
atolls in both groups, invariably consists of coral, and the lower
parts of sand. At seven or eight fathoms depth, the bottom is formed,
as could be seen through the clear water, of great living masses of
coral, which at about ten fathoms generally stand some way apart from
each other, with patches of white sand between them, and at a little
greater depth these patches become united into a smooth steep slope,
without any coral. Captain Moresby, also, informs me in support of his
statement, that he found only decayed coral on the Padua Bank (northern
part of the Laccadive group) which has an average depth between
twenty-five and thirty-five fathoms, but that on some other banks in
the same group with only ten or twelve fathoms water on them (for
instance, the Tillacapeni bank), the coral was living.
With regard to the coral-reefs in the Red Sea, Ehrenberg has the
following passage:—“The living corals do not descend there into great
depths. On the edges of islets and near reefs, where the depth was
small, very many lived; but we found no more even at six fathoms. The
pearl-fishers at Yemen and Massaua asserted that there was no coral
near the pearl-banks at nine fathoms depth, but only sand. We were not
able to institute any more special researches.”[31] I am, however,
assured both by Captain Moresby and Lieutenant Wellstead, that in the
more northern parts of the Red Sea, there are extensive beds of living
coral at a depth of twenty-five fathoms, in which the anchors of their
vessels were frequently entangled. Captain Moresby attributes the less
depth, at which the corals are able to live in the places mentioned by
Ehrenberg, to the greater quantity of sediment there; and the
situations, where they were flourishing at the depth of twenty-five
fathoms, were protected, and the water was extraordinarily limpid. On
the leeward side of Mauritius where I found the coral growing at a
somewhat greater depth than at Keeling atoll, the sea, owing apparently
to its tranquil state, was likewise very clear. Within the lagoons of
some of the Marshall atolls, where the water can be but little
agitated, there are, according to Kotzebue, living beds of coral in
twenty-five fathoms. From these facts, and considering the manner in
which the beds of clean coral off Mauritius, Keeling Island, the
Maldiva and Chagos atolls, graduated into a sandy slope, it appears
very probable that the depth, at which reef-building polypifers can
exist, is partly determined by the extent of inclined surface, which
the currents of the sea and the recoiling waves have the power to keep
free from sediment.
[31] Ehrenberg, “Über die Natür,” &c., page 50.
MM. Quoy and Gaimard[32] believe that the growth of coral is confined
within very limited depths; and they state that they never found any
fragment of an Astræa (the genus they consider most efficient in
forming reefs) at a depth above twenty-five or thirty feet. But we have
seen that in several places the bottom of the sea is paved with massive
corals at more than twice this depth; and at fifteen fathoms (or twice
this depth) off the reefs of Mauritius, the arming was marked with the
distinct impression of a living Astræa. _Millepora alcicornis_ lives in
from 0 to 12 fathoms, and the genera Madrepora and Seriatopora from 0
to 20 fathoms. Captain Moresby has given me a specimen of _Sideropora
scabra_ (Porites of Lamarck) brought up alive from 17 fathoms. Mr.
Couthouy[33] states that he has dredged up on the Bahama banks
considerable masses of Meandrina from 16 fathoms, and he has seen this
coral growing in 20 fathoms. A Caryophyllia, half an inch in diameter,
was dredged up alive from 80 fathoms off Juan Fernandez (latitude 33°
S.) by Captain P.P. King:[34] this is the most remarkable fact with
which I am acquainted, showing the depth at which a genus of corals
often found on reefs, can exist.[35] We ought, however, to feel less
surprise at this fact, as Caryophyllia alone of the lamelliform genera,
ranges far beyond the tropics; it is found in Zetland (Fleming’s
“British Animals,” genus Caryophyllia.) in Latitude 60° N. in deep
water, and I procured a small species from Tierra del Fuego in Latitude
53° S. Captain Beechey informs me, that branches of pink and yellow
coral were frequently brought up from between twenty and twenty-five
fathoms off the Low atolls; and Lieutenant Stokes, writing to me from
the N.W. coast of Australia, says that a strongly branched coral was
procured there from thirty fathoms; unfortunately it is not known to
what genera these corals belong.
[32] “Annales des Sci. Nat.” tom. vi.
[33] “Remarks on Coral Formations,” page 12.
[34] I am indebted to Mr. Stokes for having kindly communicated this
fact to me, together with much other valuable information.
[35] I will record in the form of a note all the facts that I have
been able to collect on the depths, both within and without the
tropics, at which those corals and corallines can live, which there is
no reason to suppose ever materially aid in the construction of a
reef.
NAME OF ZOOPHYTE Depth in fathoms COUNTRY and S. LATITUDE AUTHORITY
SERTULARIA 40 Cape Horn 66° Where none is given,
the observation is my own
CELLARIA ditto Ditto
CELLARIA, A minute 190 Keeling Atoll, 12°
scarlet encrusting
species found living
CELLARIA, An allied, 48 St Cruz Riv. 50°
small stony sub-generic
form
A coral allied to 40 Cape Horn
VINCULARIA, with eight
rows of cells
TUBULIPORA, near to 40 Cape Horn
T. patina
TUBULIPORA, near to 94 East Chiloe 43°
T. patina
CELLEPORA, several 40 Cape Horn
species, and allied
sub-generic forms
CELLEPORA, several 40 and 57 Chonos Archipelago 45°
species, and allied
sub-generic forms
CELLEPORA, several 48 St Cruz 50°
species, and allied
sub-generic forms
ESCHARA 30 Tierra del Fuego 53°
ESCHARA 48 St Cruz R. 50°
RETEPORA 40 Cape Horn
RETEPORA 100 Cape of Good Hope 34° Quoy and Gaimard, “Ann. Scien. Nat.” tome vi., page 284.
MILLEPORA, a strong 94 and 30 E. Chiloe 43°
coral with cylindrical
branches, of a pink
colour, about two
inches high,
resembling in the
form of its orifices
M. aspera of Lamarck
Tierra del Fuego 53°
CORALIUM 120 Barbary 33° N. Peyssonel in paper read to Royal Society May 1752.
ANTIPATHES 16 Chonos 45°
GORGONIA (or an allied 160 Abrolhos on the Captain Beechey informed me of this fact in a letter.
form) coast of Brazil 18°
Ellis (“Nat. Hist. of Coralline,” page 96) states that Ombellularia was
procured in latitude 79° N. _sticking_ to a _line_ from the depth of
236 fathoms; hence this coral either must have been floating loose, or
was entangled in stray line at the bottom. Off Keeling atoll a compound
Ascidia (Sigillina) was brought up from 39 fathoms, and a piece of
sponge, apparently living, from 70, and a fragment of Nullipora also
apparently living from 92 fathoms. At a greater depth than 90 fathoms
off this coral island, the bottom was thickly strewed with joints of
Halimeda and small fragments of other Nulliporæ, but all dead. Captain
B. Allen, R.N., informs me that in the survey of the West Indies it was
noticed that between the depth of 10 and 200 fathoms, the sounding lead
very generally came up coated with the dead joints of a Halimeda, of
which he showed me specimens. Off Pernambuco, in Brazil, in about
twelve fathoms, the bottom was covered with fragments dead and alive of
a dull red Nullipora, and I infer from Roussin’s chart, that a bottom
of this kind extends over a wide area. On the beach, within the
coral-reefs of Mauritius, vast quantities of fragments of Nulliporæ
were piled up. From these facts it appears, that these simply organized
bodies are amongst the most abundant productions of the sea.)
Although the limit of depth, at which each particular kind of coral
ceases to exist, is far from being accurately known; yet when we bear
in mind the manner in which the clumps of coral gradually became
infrequent at about the same depth, and wholly disappeared at a greater
depth than twenty fathoms, on the slope round Keeling atoll, on the
leeward side of the Mauritius, and at rather less depth, both without
and within the atolls of the Maldiva and Chagos Archipelagoes; and when
we know that the reefs round these islands do not differ from other
coral formations in their form and structure, we may, I think, conclude
that in ordinary cases, reef-building polypifers do not flourish at
greater depths than between twenty and thirty fathoms.
It has been argued[36] that reefs may possibly rise from very great
depths through the means of small corals, first making a platform for
the growth of the stronger kinds. This, however, is an arbitrary
supposition: it is not always remembered, that in such cases there is
an antagonist power in action, namely, the decay of organic bodies,
when not protected by a covering of sediment, or by their own rapid
growth. We have, moreover, no right to calculate on unlimited time for
the accumulation of small organic bodies into great masses. Every fact
in geology proclaims that neither the land, nor the bed of the sea
retain for indefinite periods the same level. As well might it be
imagined that the British Seas would in time become choked up with beds
of oysters, or that the numerous small corallines off the inhospitable
shores of Tierra del Fuego would in time form a solid and extensive
coral-reef.
[36] “Journal of the Royal Geographical Society,” 1831, page 218.
CHAPTER V.
THEORY OF THE FORMATION OF THE DIFFERENT CLASSES OF CORAL-REEFS.
The atolls of the larger archipelagoes are not formed on submerged
craters, or on banks of sediment.—Immense areas interspersed with
atolls.—Their subsidence.—The effects of storms and earthquakes on
atolls.—Recent changes in their state.—The origin of barrier-reefs and
of atolls.—Their relative forms.—The step-formed ledges and walls round
the shores of some lagoons.—The ring-formed reefs of the Maldiva
atolls.—The submerged condition of parts or of the whole of some
annular reefs.—The disseverment of large atolls.—The union of atolls by
linear reefs.—The Great Chagos Bank.—Objections from the area and
amount of subsidence required by the theory, considered.—The probable
composition of the lower parts of atolls.
The naturalists who have visited the Pacific, seem to have had their
attention riveted by the lagoon-islands, or atolls,—those singular
rings of coral-land which rise abruptly out of the unfathomable
ocean—and have passed over, almost unnoticed, the scarcely less
wonderful encircling barrier-reefs. The theory most generally received
on the formation of atolls, is that they are based on submarine
craters; but where can we find a crater of the shape of Bow atoll,
which is five times as long as it is broad (Plate I., Figure 4); or
like that of Menchikoff Island (Plate II., Figure 3.), with its three
loops, together sixty miles in length; or like Rimsky Korsacoff,
narrow, crooked, and fifty-four miles long; or like the northern
Maldiva atolls, made up of numerous ring-formed reefs, placed on the
margin of a disc,—one of which discs is eighty-eight miles in length,
and only from ten to twenty in breadth? It is, also, not a little
improbable, that there should have existed as many craters of immense
size crowded together beneath the sea, as there are now in some parts
atolls. But this theory lies under a greater difficulty, as will be
evident, when we consider on what foundations the atolls of the larger
archipelagoes rest: nevertheless, if the rim of a crater afforded a
basis at the proper depth, I am far from denying that a reef like a
perfectly characterised atoll might not be formed; some such, perhaps,
now exist; but I cannot believe in the possibility of the greater
number having thus originated.
An earlier and better theory was proposed by Chamisso;[1] he supposes
that as the more massive kinds of corals prefer the surf, the outer
portions, in a reef rising from a submarine basis, would first reach
the surface and consequently form a ring. But on this view it must be
assumed, that in every case the basis consists of a flat bank; for if
it were conically formed, like a mountainous mass, we can see no reason
why the coral should spring up from the flanks, instead of from the
central and highest parts: considering the number of the atolls in the
Pacific and Indian Oceans, this assumption is very improbable. As the
lagoons of atolls are sometimes even more than forty fathoms deep, it
must, also, be assumed on this view, that at a depth at which the waves
do not break, the coral grows more vigorously on the edges of a bank
than on its central part; and this is an assumption without any
evidence in support of it. I remarked, in the third chapter, that a
reef, growing on a detached bank, would tend to assume an atoll-like
structure; if, therefore, corals were to grow up from a bank, with a
level surface some fathoms submerged, having steep sides and being
situated in a deep sea, a reef not to be distinguished from an atoll,
might be formed: I believe some such exist in the West Indies. But a
difficulty of the same kind with that affecting the crater theory,
runners, as we shall presently see, this view inapplicable to the
greater number of atolls.
[1] Kotzebue’s “First Voyage,” volume iii., page 331.
No theory worthy of notice has been advanced to account for those
barrier-reefs, which encircle islands of moderate dimensions. The great
reef which fronts the coast of Australia has been supposed, but without
any special facts, to rest on the edge of a submarine precipice,
extending parallel to the shore. The origin of the third class or of
fringing-reefs presents, I believe, scarcely any difficulty, and is
simply consequent on the polypifers not growing up from great depths,
and their not flourishing close to gently shelving beaches where the
water is often turbid.
What cause, then, has given to atolls and barrier-reefs their
characteristic forms? Let us see whether an important deduction will
not follow from the consideration of these two circumstances, first,
the reef-building corals flourishing only at limited depths; and
secondly, the vastness of the areas interspersed with coral-reefs and
coral-islets, none of which rise to a greater height above the level of
the sea, than that attained by matter thrown up by the waves and winds.
I do not make this latter statement vaguely; I have carefully sought
for descriptions of every island in the intertropical seas; and my task
has been in some degree abridged by a map of the Pacific, corrected in
1834 by MM. D’Urville and Lottin, in which the low islands are
distinguished from the high ones (even from those much less than a
hundred feet in height) by being written without a capital letter; I
have detected a few errors in this map, respecting the height of some
of the islands, which will be noticed in the Appendix, where I treat of
coral formations in geographical order. To the Appendix, also, I must
refer for a more particular account of the data on which the statements
on the next page are grounded. I have ascertained, and chiefly from the
writings of Cook, Kotzebue, Bellinghausen, Duperrey, Beechey, and
Lutké, regarding the Pacific; and from Moresby[2] with respect to the
Indian Ocean, that in the following cases the term “low island”
strictly means land of the height commonly attained by matter thrown up
by the winds and the waves of an open sea. If we draw a line (the plan
I have always adopted) joining the external atolls of that part of the
Low Archipelago in which the islands are numerous, the figure will be a
pointed ellipse (reaching from Hood to Lazaref Island), of which the
longer axis is 840 geographical miles, and the shorter 420 miles; in
this space[3] none of the innumerable islets united into great rings
rise above the stated level. The Gilbert group is very narrow, and 300
miles in length. In a prolonged line from this group, at the distance
of 240 miles, is the Marshall Archipelago, the figure of which is an
irregular square, one end being broader than the other; its length is
520 miles, with an average width of 240; these two groups together are
1,040 miles in length, and all their islets are low. Between the
southern end of the Gilbert and the northern end of Low Archipelago,
the ocean is thinly strewed with islands, all of which, as far as I
have been able to ascertain, are low; so that from nearly the southern
end of the Low Archipelago, to the northern end of the Marshall
Archipelago, there is a narrow band of ocean, more than 4,000 miles in
length, containing a great number of islands, all of which are low. In
the western part of the Caroline Archipelago, there is a space of 480
miles in length, and about 100 broad, thinly interspersed with low
islands. Lastly, in the Indian Ocean, the archipelago of the Maldivas
is 470 miles in length, and 60 in breadth; that of the Laccadives is
150 by 100 miles; as there is a low island between these two groups,
they may be considered as one group of 1,000 miles in length. To this
may be added the Chagos group of low islands, situated 280 miles
distant, in a line prolonged from the southern extremity of the
Maldivas. This group, including the submerged banks, is 170 miles in
length and 80 in breadth. So striking is the uniformity in direction of
these three archipelagoes, all the islands of which are low, that
Captain Moresby, in one of his papers, speaks of them as parts of one
great chain, nearly 1,500 miles long. I am, then, fully justified in
repeating, that enormous spaces, both in the Pacific and Indian Oceans,
are interspersed with islands, of which not one rises above that
height, to which the waves and winds in an open sea can heap up matter.
[2] See also Captain Owen’s and Lieutenant Wood’s papers in the
“Geographical Journal”, on the Maldiva and Laccadive Archipelagoes.
These officers particularly refer to the lowness of the islets; but I
chiefly ground my assertion respecting these two groups, and the
Chagos group, from information communicated to me by Captain Moresby.
[3] I find from Mr. Couthouy’s pamphlet (page 58) that Aurora Island
is about two hundred feet in height; it consists of coral-rock, and
seems to have been formed by the elevation of an atoll. It lies
north-east of Tahiti, close without the line bounding the space
coloured dark blue in the map appended to this volume. Honden Island,
which is situated in the extreme north-west part of the Low
Archipelago, according to measurements made on board the _Beagle_,
whilst sailing by, is 114 feet from the _summit of the trees_ to the
water’s edge. This island appeared to resemble the other atolls of the
group.
On what foundations, then, have these reefs and islets of coral been
constructed? A foundation must originally have been present beneath
each atoll at that limited depth, which is indispensable for the first
growth of the reef-building polypifers. A conjecture will perhaps be
hazarded, that the requisite bases might have been afforded by the
accumulation of great banks of sediment, which owing to the action of
superficial currents (aided possibly by the undulatory movement of the
sea) did not quite reach the surface,—as actually appears to have been
the case in some parts of the West Indian Sea. But in the form and
disposition of the groups of atolls, there is nothing to countenance
this notion; and the assumption without any proof, that a number of
immense piles of sediment have been heaped on the floor of the great
Pacific and Indian Oceans, in their central parts far remote from land,
and where the dark blue colour of the limpid water bespeaks its purity,
cannot for one moment be admitted.
The many widely-scattered atolls must, therefore, rest on rocky bases.
But we cannot believe that the broad summit of a mountain lies buried
at the depth of a few fathoms beneath every atoll, and nevertheless
throughout the immense areas above-named, with not one point of rock
projecting above the level of the sea; for we may judge with some
accuracy of mountains beneath the sea, by those on the land; and where
can we find a single chain several hundred miles in length and of
considerable breadth, much less several such chains, with their many
broad summits attaining the same height, within from 120 to 180 feet?
If the data be thought insufficient, on which I have grounded my
belief, respecting the depth at which the reef-building polypifers can
exist, and it be assumed that they can flourish at a depth of even one
hundred fathoms, yet the weight of the above argument is but little
diminished, for it is almost equally improbable, that as many submarine
mountains, as there are low islands in the several great and widely
separated areas above specified, should all rise within six hundred
feet of the surface of the sea and not one above it, as that they
should be of the same height within the smaller limit of one or two
hundred feet. So highly improbable is this supposition, that we are
compelled to believe, that the bases of the many atolls did never at
any one period all lie submerged within the depth of a few fathoms
beneath the surface, but that they were brought into the requisite
position or level, some at one period and some at another, through
movements in the earth’s crust. But this could not have been effected
by elevation, for the belief that points so numerous and so widely
separated were successively uplifted to a certain level, but that not
one point was raised above that level, is quite as improbable as the
former supposition, and indeed differs little from it. It will probably
occur to those who have read Ehrenberg’s account of the Reefs of the
Red Sea, that many points in these great areas may have been elevated,
but that as soon as raised, the protuberant parts were cut off by the
destroying action of the waves: a moment’s reflection, however, on the
basin-like form of the atolls, will show that this is impossible; for
the upheaval and subsequent abrasion of an island would leave a flat
disc, which might become coated with coral, but not a deeply concave
surface; moreover, we should expect to see, in some parts at least, the
rock of the foundation brought to the surface. If, then, the
foundations of the many atolls were not uplifted into the requisite
position, they must of necessity have subsided into it; and this at
once solves every difficulty,[4] for we may safely infer, from the
facts given in the last chapter, that during a gradual subsidence the
corals would be favourably circumstanced for building up their solid
frame works and reaching the surface, as island after island slowly
disappeared. Thus areas of immense extent in the central and most
profound parts of the great oceans, might become interspersed with
coral-islets, none of which would rise to a greater height than that
attained by detritus heaped up by the sea, and nevertheless they might
all have been formed by corals, which absolutely required for their
growth a solid foundation within a few fathoms of the surface.
[4] The additional difficulty on the crater hypothesis before alluded
to, will now be evident; for on this view the volcanic action must be
supposed to have formed within the areas specified a vast number of
craters, all rising within a few fathoms of the surface, and not one
above it. The supposition that the craters were at different times
upraised above the surface, and were there abraded by the surf and
subsequently coated by corals, is subject to nearly the same
objections with those given above in this paragraph; but I consider it
superfluous to detail all the arguments opposed to such a notion.
Chamisso’s theory, from assuming the existence of so many banks, all
lying at the proper depth beneath the water, is also vitally
defective. The same observation applies to an hypothesis of Lieutenant
Nelson’s (“Geolog. Trans.” volume v., page 122), who supposes that the
ring-formed structure is caused by a greater number of germs of corals
becoming attached to the declivity, than to the central plateau of a
submarine bank: it likewise applies to the notion formerly entertained
(Forster’s “Observ.” page 151), that lagoon-islands owe their peculiar
form to the instinctive tendencies of the polypifers. According to
this latter view, the corals on the outer margin of the reef
instinctively expose themselves to the surf in order to afford
protection to corals living in the lagoon, which belong to other
genera, and to other families!
It would be out of place here to do more than allude to the many facts,
showing that the supposition of a gradual subsidence over large areas
is by no means improbable. We have the clearest proof that a movement
of this kind is possible, in the upright trees buried under the strata
many thousand feet in thickness; we have also every reason for
believing that there are now large areas gradually sinking, in the same
manner as others are rising. And when we consider how many parts of the
surface of the globe have been elevated within recent geological
periods, we must admit that there have been subsidences on a
corresponding scale, for otherwise the whole globe would have swollen.
It is very remarkable that Mr. Lyell,[5] even in the first edition of
his “Principles of Geology,” inferred that the amount of subsidence in
the Pacific must have exceeded that of elevation, from the area of land
being very small relatively to the agents there tending to form it,
namely, the growth of coral and volcanic action. But it will be asked,
are there any direct proofs of a subsiding movement in those areas, in
which subsidence will explain a phenomenon otherwise inexplicable?
This, however, can hardly be expected, for it must ever be most
difficult, excepting in countries long civilised, to detect a movement,
the tendency of which is to conceal the part affected. In barbarous and
semi-civilised nations how long might not a slow movement, even of
elevation such as that now affecting Scandinavia, have escaped
attention!
[5] “Principles of Geology,” sixth edition, volume iii., page 386.
Mr. Williams[6] insists strongly that the traditions of the natives,
which he has taken much pains in collecting, do not indicate the
appearance of any new islands: but on the theory of a gradual
subsidence, all that would be apparent would be, the water sometimes
encroaching slowly on the land, and the land again recovering by the
accumulation of detritus its former extent, and perhaps sometimes the
conversion of an atoll with coral islets on it, into a bare or into a
sunken annular reef. Such changes would naturally take place at the
periods when the sea rose above its usual limits, during a gale of more
than ordinary strength; and the effects of the two causes would be
hardly distinguishable. In Kotzebue’s “Voyage” there are accounts of
islands, both in the Caroline and Marshall Archipelagoes, which have
been partly washed away during hurricanes; and Kadu, the native who was
on board one of the Russian vessels, said “he saw the sea at Radack
rise to the feet of the cocoa-nut trees; but it was conjured in
time.”[7] A storm lately entirely swept away two of the Caroline
islands, and converted them into shoals; it partly, also, destroyed two
other islands.[8] According to a tradition which was communicated to
Captain Fitzroy, it is believed in the Low Archipelago, that the
arrival of the first ship caused a great inundation, which destroyed
many lives. Mr. Stutchbury relates, that in 1825, the western side of
Chain Atoll, in the same group, was completely devastated by a
hurricane, and not less than 300 lives lost: “in this instance it was
evident, even to the natives, that the hurricane alone was not
sufficient to account for the violent agitation of the ocean.”[9] That
considerable changes have taken place recently in some of the atolls in
the Low Archipelago, appears certain from the case already given of
Matilda Island: with respect to Whitsunday and Gloucester Islands in
this same group, we must either attribute great inaccuracy to their
discoverer, the famous circumnavigator Wallis, or believe that they
have undergone a considerable change in the period of fifty-nine years,
between his voyage and that of Captain Beechey’s. Whitsunday Island is
described by Wallis as “about four miles long, and three wide,” now it
is only one mile and a half long. The appearance of Gloucester Island,
in Captain Beechey’s words,[10] “has been accurately described by its
discoverer, but its present form and extent differ materially.”
Blenheim reef, in the Chagos group, consists of a water-washed annular
reef, thirteen miles in circumference, surrounding a lagoon ten fathoms
deep: on its surface there were a few worn patches of conglomerate
coral-rock, of about the size of hovels; and these Captain Moresby
considered as being, without doubt, the last remnants of islets; so
that here an atoll has been converted into an atoll-formed reef. The
inhabitants of the Maldiva Archipelago, as long ago as 1605, declared,
“that the high tides and violent currents were diminishing the number
of the islands:”[11] and I have already shown, on the authority of
Captain Moresby, that the work of destruction is still in progress; but
that on the other hand the first formation of some islets is known to
the present inhabitants. In such cases, it would be exceedingly
difficult to detect a gradual subsidence of the foundation, on which
these mutable structures rest.
[6] Williams’s “Narrative of Missionary Enterprise,” page 31.
[7] Kotzebue’s “First Voyage,” volume iii., page 168.
[8] M. Desmoulins in “Comptes Rendus,” 1840, page 837.
[9] “West of England Journal”, No. I., page 35.
[10] Beechey’s “Voyage to the Pacific,” chapter vii., and Wallis’s
“Voyage in the ‘Dolphin’,” chapter iv.
[11] See an extract from Pyrard’s Voyage in Captain Owen’s paper on
the Maldiva Archipelago, in the “Geographical Journal”, volume ii.,
page 84.
Some of the archipelagoes of low coral-islands are subject to
earthquakes: Captain Moresby informs me that they are frequent, though
not very strong, in the Chagos group, which occupies a very central
position in the Indian Ocean, and is far from any land not of coral
formation. One of the islands in this group was formerly covered by a
bed of mould, which, after an earthquake, disappeared, and was believed
by the residents to have been washed by the rain through the broken
masses of underlying rock; the island was thus rendered unproductive.
Chamisso[12] states, that earthquakes are felt in the Marshall atolls,
which are far from any high land, and likewise in the islands of the
Caroline Archipelago. On one of the latter, namely Oulleay atoll,
Admiral Lutké, as he had the kindness to inform me, observed several
straight fissures about a foot in width, running for some hundred yards
obliquely across the whole width of the reef. Fissures indicate a
stretching of the earth’s crust, and, therefore, probably changes in
its level; but these coral-islands, which have been shaken and
fissured, certainly have not been elevated, and, therefore, probably
they have subsided. In the chapter on Keeling atoll, I attempted to
show by direct evidence, that the island underwent a movement of
subsidence, during the earthquakes lately felt there.
[12] See Chamisso, in Kotzebue’s “First Voyage,” volume iii., pages
182 and 136.
The facts stand thus;—there are many large tracts of ocean, without any
high land, interspersed with reefs and islets, formed by the growth of
those kinds of corals, which cannot live at great depths; and the
existence of these reefs and low islets, in such numbers and at such
distant points, is quite inexplicable, excepting on the theory, that
the bases on which the reefs first became attached, slowly and
successively sank beneath the level of the sea, whilst the corals
continued to grow upwards. No positive facts are opposed to this view,
and some general considerations render it probable. There is evidence
of change in form, whether or not from subsidence, on some of these
coral-islands; and there is evidence of subterranean disturbances
beneath them. Will then the theory, to which we have thus been led,
solve the curious problem,—what has given to each class of reef its
peculiar form?
[Illustration]
AA—Outer edge of the reef at the level of the sea.
BB—Shores of the island.
A′A′—Outer edge of the reef, after its upward growth during a period of
subsidence.
CC—The lagoon-channel between the reef and the shores of the now
encircled land.
B′B′—The shores of the encircled island.
N.B.—In this, and the following woodcut, the subsidence of the land
could only be represented by an apparent rise in the level of the sea.
[Illustration]
A′A′—Outer edges of the barrier-reef at the level of the sea. The
cocoa-nut trees represent coral-islets formed on the reef.
CC—The lagoon-channel.
B′B′—The shores of the island, generally formed of low alluvial land
and of coral detritus from the lagoon-channel.
A″A″—The outer edges of the reef now forming an atoll.
C′—The lagoon of the newly formed atoll. According to the scale, the
depth of the lagoon and of the lagoon-channel is exaggerated.
Let us in imagination place within one of the subsiding areas, an
island surrounded by a “fringing-reef,”—that kind, which alone offers
no difficulty in the explanation of its origin. Let the unbroken lines
and the oblique shading in the woodcut (No. 4) represent a vertical
section through such an island; and the horizontal shading will
represent the section of the reef. Now, as the island sinks down,
either a few feet at a time or quite insensibly, we may safely infer
from what we know of the conditions favourable to the growth of coral,
that the living masses bathed by the surf on the margin of the reef,
will soon regain the surface. The water, however, will encroach, little
by little, on the shore, the island becoming lower and smaller, and the
space between the edge of the reef and the beach proportionately
broader. A section of the reef and island in this state, after a
subsidence of several hundred feet, is given by the dotted lines:
coral-islets are supposed to have been formed on the new reef, and a
ship is anchored in the lagoon-channel. This section is in every
respect that of an encircling barrier-reef; it is, in fact, a section
taken[13] E. and W. through the highest point of the encircled island
of Bolabola; of which a plan is given in Plate I., Figure 5. The same
section is more clearly shown in the following woodcut (No. 5) by the
unbroken lines. The width of the reef, and its slope, both on the outer
and inner side, will have been determined by the growing powers of the
coral, under the conditions (for instance the force of the breakers and
of the currents) to which it has been exposed; and the lagoon-channel
will be deeper or shallower, in proportion to the growth of the
delicately branched corals within the reef, and to the accumulation of
sediment, relatively, also, to the rate of subsidence and the length of
the intervening stationary periods.
[13] The section has been made from the chart given in the “Atlas of
the Voyage of the _Coquille_.” The scale is .57 of an inch to a mile.
The height of the island, according to M. Lesson, is 4,026 feet. The
deepest part of the lagoon-channel is 162 feet; its depth is
exaggerated in the woodcut for the sake of clearness.
It is evident in this section, that a line drawn perpendicularly down
from the outer edge of the new reef to the foundation of solid rock,
exceeds by as many feet as there have been feet of subsidence, that
small limit of depth at which the effective polypifers can live—the
corals having grown up, as the whole sank down, from a basis formed of
other corals and their consolidated fragments. Thus the difficulty on
this head, which before seemed so great, disappears.
As the space between the reef and the subsiding shore continued to
increase in breadth and depth, and as the injurious effects of the
sediment and fresh water borne down from the land were consequently
lessened, the greater number of the channels, with which the reef in
its fringing state must have been breached, especially those which
fronted the smaller streams, will have become choked up with the growth
of coral: on the windward side of the reef, where the coral grows most
vigorously, the breaches will probably have first been closed. In
barrier-reefs, therefore, the breaches kept open by draining the tidal
waters of the lagoon-channel, will generally be placed on the leeward
side, and they will still face the mouths of the larger streams,
although removed beyond the influence of their sediment and fresh
water;—and this, it has been shown, is commonly the case.
Referring to the diagram shown above, in which the newly formed
barrier-reef is represented by unbroken lines, instead of by dots as in
the former woodcut, let the work of subsidence go on, and the doubly
pointed hill will form two small islands (or more, according to the
number of the hills) included within one annular reef. Let the island
continue subsiding, and the coral-reef will continue growing up on its
own foundation, whilst the water gains inch by inch on the land, until
the last and highest pinnacle is covered, and there remains a perfect
atoll. A vertical section of this atoll is shown in the woodcut by the
dotted lines;—a ship is anchored in its lagoon, but islets are not
supposed yet to have been formed on the reef. The depth of the lagoon
and the width and slope of the reef, will depend on the circumstances
just referred to under barrier-reefs. Any further subsidence will
produce no change in the atoll, except perhaps a diminution in its
size, from the reef not growing vertically upwards; but should the
currents of the sea act violently upon it, and should the corals perish
on part or on the whole of its margin, changes would result during
subsidence which will be presently noticed. I may here observe, that a
bank either of rock or of hardened sediment, level with the surface of
the sea, and fringed with living coral, would (if not so small as to
allow the central space to be quickly filled up with detritus) by
subsidence be converted immediately into an atoll, without passing, as
in the case of a reef fringing the shore of an island, through the
intermediate form of a barrier-reef. If such a bank lay a few fathoms
submerged, the simple growth of the coral (as remarked in the third
chapter) without the aid of subsidence, would produce a structure
scarcely to be distinguished from a true atoll; for in all cases the
corals on the outer margin of a reef, from having space and being
freely exposed to the open sea, will grow vigorously and tend to form a
continuous ring whilst the growth of the less massive kinds on the
central expanse, will be checked by the sediment formed there, and by
that washed inwards by the breakers; and as the space becomes
shallower, their growth will, also, be checked by the impurities of the
water, and probably by the small amount of food brought by the
enfeebled currents, in proportion to the surface of living reefs
studded with innumerable craving mouths: the subsidence of a reef based
on a bank of this kind, would give depth to its central expanse or
lagoon, steepness to its flanks, and through the free growth of the
coral, symmetry to its outline:—I may here repeat that the larger
groups of atolls in the Pacific and Indian Oceans cannot be supposed to
be founded on banks of this nature.
If, instead of the island in the diagram, the shore of a continent
fringed by a reef had subsided, a great barrier-reef, like that on the
north-east coast of Australia, would have necessarily resulted; and it
would have been separated from the main land by a deep-water channel,
broad in proportion to the amount of subsidence, and to the less or
greater inclination of the neighbouring coast-line. The effect of the
continued subsidence of a great barrier-reef of this kind, and its
probable conversion into a chain of separate atolls, will be noticed,
when we discuss the apparent progressive disseverment of the larger
Maldiva atolls.
We now are able to perceive that the close similarity in form,
dimensions, structure, and relative position (which latter point will
hereafter be more fully noticed) between fringing and encircling
barrier-reefs, and between these latter and atolls, is the necessary
result of the transformation, during subsidence of the one class into
the other. On this view, the three classes of reefs ought to graduate
into each other. Reefs having intermediate character between those of
the fringing and barrier classes do exist; for instance, on the
south-west coast of Madagascar, a reef extends for several miles,
within which there is a broad channel from seven to eight fathoms deep,
but the sea does not deepen abruptly outside the reef. Such cases,
however, are open to some doubts, for an old fringing-reef, which had
extended itself a little on a basis of its own formation, would hardly
be distinguishable from a barrier-reef, produced by a small amount of
subsidence, and with its lagoon-channel nearly filled up with sediment
during a long stationary period. Between barrier-reefs, encircling
either one lofty island or several small low ones, and atolls including
a mere expanse of water, a striking series can be shown: in proof of
this, I need only refer to the first plate in this volume, which speaks
more plainly to the eye, than any description could to the ear. The
authorities from which the charts have been engraved, together with
some remarks on them and descriptive of the plates, are given above. At
New Caledonia (Plate II., Figure 5.) the barrier-reefs extend for 150
miles on each side of the submarine prolongation of the island; and at
their northern extremity they appear broken up and converted into a
vast atoll-formed reef, supporting a few low coral-islets: we may
imagine that we here see the effects of subsidence actually in
progress, the water always encroaching on the northern end of the
island, towards which the mountains slope down, and the reefs steadily
building up their massive fabrics in the lines of their ancient growth.
We have as yet only considered the origin of barrier-reefs and atolls
in their simplest form; but there remain some peculiarities in
structure and some special cases, described in the two first chapters,
to be accounted for by our theory. These consist—in the inclined ledge
terminated by a wall, and sometimes succeeded by a second ledge with a
wall, round the shores of certain lagoons and lagoon-channels; a
structure which cannot, as I endeavoured to show, be explained by the
simple growing powers of the corals,—in the ring or basin-like forms of
the central reefs, as well as of the separate marginal portions of the
northern Maldiva atolls,—in the submerged condition of the whole, or of
parts of certain barrier and atoll-formed reefs; where only a part is
submerged, this being generally to leeward,—in the apparent progressive
disseverment of some of the Maldiva atolls,—in the existence of
irregularly formed atolls, some being tied together by linear reefs,
and others with spurs projecting from them,—and, lastly, in the
structure and origin of the Great Chagos bank.
_Step-formed ledges round certain lagoons_.—If we suppose an atoll to
subside at an extremely slow rate, it is difficult to follow out the
complex results. The living corals would grow up on the outer margin;
and likewise probably in the gullies and deeper parts of the bare
surface of the annular reef; the water would encroach on the islets,
but the accumulation of fresh detritus might possibly prevent their
entire submergence. After a subsidence of this very slow nature, the
surface of the annular reef sloping gently into the lagoon, would
probably become united with the irregular reefs and banks of sand,
which line the shores of most lagoons. Should, however, the atoll be
carried down by a more rapid movement, the whole surface of the annular
reef, where there was a foundation of solid matter, would be favourably
circumstanced for the fresh growth of coral; but as the corals grew
upwards on its exterior margin, and the waves broke heavily on this
part, the increase of the massive polypifers on the inner side would be
checked from the want of water. Consequently, the exterior parts would
first reach the surface, and the new annular reef thus formed on the
old one, would have its summit inclined inwards, and be terminated by a
subaqueous wall, formed by the upward growth of the coral (before being
much checked), from the inner edge of the solid parts of the old reef.
The inner portion of the new reef, from not having grown to the
surface, would be covered by the waters of the lagoon. Should a
subsidence of the same kind be repeated, the corals would again grow up
in a wall, from all the solid parts of the resunken reef, and,
therefore, not from within the sandy shores of the lagoon; and the
inner part of the new annular reef would, from being as before checked
in its upward growth, be of less height than the exterior parts, and
therefore would not reach the surface of the lagoon. In this case the
shores of the lagoon would be surrounded by two inclined ledges, one
beneath the other, and both abruptly terminated by subaqueous
cliffs.[14]
[14] According to Mr. Couthouy (page 26) the external reef round many
atolls descends by a succession of ledges or terraces. He attempts, I
doubt whether successfully, to explain this structure somewhat in the
same manner as I have attempted, with respect to the internal ledges
round the lagoons of some atolls. More facts are wanted regarding the
nature both of the interior and exterior step-like ledges: are all the
ledges, or only the upper ones, covered with living coral? If they are
all covered, are the kinds different on the ledges according to the
depth? Do the interior and exterior ledges occur together in the same
atolls; if so, what is their total width, and is the intervening
surface-reef narrow, etc.?
_The ring or basin-formed reefs of the northern Maldiva atolls_.—I may
first observe, that the reefs within the lagoons of atolls and within
lagoon-channels, would, if favourably circumstanced, grow upwards
during subsidence in the same manner as the annular rim; and,
therefore, we might expect that such lagoon-reefs, when not surrounded
and buried by an accumulation of sediment more rapid than the rate of
subsidence, would rise abruptly from a greater depth than that at which
the efficient polypifers can flourish: we see this well exemplified in
the small abruptly-sided reefs, with which the deep lagoons of the
Chagos and Southern Maldiva atolls are studded. With respect to the
ring or basin-formed reefs of the Northern Maldiva atolls, it is
evident, from the perfectly continuous series which exists that the
marginal rings, although wider than the exterior or bounding reef of
ordinary atolls, are only modified portions of such a reef; it is also
evident that the central rings, although wider than the knolls or reefs
which commonly occur in lagoons, occupy their place. The ring-like
structure has been shown to be contingent on the breaches into the
lagoon being broad and numerous, so that all the reefs which are bathed
by the waters of the lagoon are placed under nearly the same conditions
with the outer coast of an atoll standing in the open sea. Hence the
exterior and living margins of these reefs must have been favourably
circumstanced for growing outwards, and increasing beyond the usual
breadth; and they must likewise have been favourably circumstanced for
growing vigorously upwards, during the subsiding movements, to which by
our theory the whole archipelago has been subjected; and subsidence
with this upward growth of the margins would convert the central space
of each little reef into a small lagoon. This, however, could only take
place with those reefs, which had increased to a breadth sufficient to
prevent their central spaces from being almost immediately filled up
with the sand and detritus driven inwards from all sides: hence it is
that few reefs, which are less than half a mile in diameter, even in
the atolls where the basin-like structure is most strikingly exhibited,
include lagoons. This remark, I may add, applies to all coral-reefs
wherever found. The basin-formed reefs of the Maldiva Archipelago may,
in fact, be briefly described, as small atolls formed during subsidence
over the separate portions of large and broken atolls, in the same
manner as these latter were formed over the barrier-reefs, which
encircled the islands of a large archipelago now wholly submerged.
_Submerged and Dead Reefs_.—In the second section of the first chapter,
I have shown that there are in the neighbourhood of atolls, some deeply
submerged banks, with level surfaces; that there are others, less
deeply but yet wholly submerged, having all the characters of perfect
atolls, but consisting merely of dead coral-rock; that there are
barrier-reefs and atolls with merely a portion of their reef, generally
on the leeward side, submerged; and that such portions either retain
their perfect outline, or they appear to be quite effaced, their former
place being marked only by a bank, conforming in outline with that part
of the reef which remains perfect. These several cases are, I believe,
intimately related together, and can be explained by the same means.
There, perhaps, exist some submerged reefs, covered with living coral
and growing upwards, but to these I do not here refer.
As we see that in those parts of the ocean, where coral-reefs are most
abundant, one island is fringed and another neighbouring one is not
fringed; as we see in the same archipelago, that all the reefs are more
perfect in one part of it than in another, for instance, in the
southern half compared with the northern half of the Maldiva
Archipelago, and likewise on the outer coasts compared with the inner
coasts of the atolls in this same group, which are placed in a double
row; as we know that the existence of the innumerable polypifers
forming a reef, depends on their sustenance, and that they are preyed
on by other organic beings; and, lastly, as we know that some inorganic
causes are highly injurious to the growth of coral, it cannot be
expected that during the round of change to which earth, air, and water
are exposed, the reef-building polypifers should keep alive for
perpetuity in any one place; and still less can this be expected,
during the progressive subsidences, perhaps at some periods more rapid
than at others, to which by our theory these reefs and islands have
been subjected and are liable. It is, then, not improbable that the
corals should sometimes perish either on the whole or on part of a
reef; if on part, the dead portion, after a small amount of subsidence,
would still retain its proper outline and position beneath the water.
After a more prolonged subsidence, it would probably form, owing to the
accumulation of sediment, only the margin of a flat bank, marking the
limits of the former lagoon. Such dead portions of reef would generally
lie on the leeward side,[15] for the impure water and fine sediment
would more easily flow out from the lagoon over this side of the reef,
where the force of the breakers is less than to windward; and therefore
the corals would be less vigorous on this side, and be less able to
resist any destroying agent. It is likewise owing to this same cause,
that reefs are more frequently breached to leeward by narrow channels,
serving as by ship-channels, than to windward. If the corals perished
entirely, or on the greater part of the circumference of an atoll, an
atoll-shaped bank of dead rock, more or less entirely submerged, would
be produced; and further subsidence, together with the accumulation of
sediment, would often obliterate its atoll-like structure, and leave
only a bank with a level surface.
[15] Mr. Lyell, in the first edition of his “Principles of Geology,”
offered a somewhat different explanation of this structure. He
supposes that there has been subsidence; but he was not aware that the
submerged portions of reef were in most cases, if not in all, dead;
and he attributes the difference in height in the two sides of most
atolls, chiefly to the greater accumulation of detritus to windward
than to leeward. But as matter is accumulated only on the backward
part of the reef, the front part would remain of the same height on
both sides. I may here observe that in most cases (for instance, at
Peros Banhos, the Gambier group and the Great Chagos Bank), and I
suspect in all cases, the dead and submerged portions do not blend or
slope into the living and perfect parts, but are separated from them
by an abrupt line. In some instances small patches of living reef rise
to the surface from the middle of the submerged and dead parts.
In the Chagos group of atolls, within an area of 160 miles by 60, there
are two atoll-formed banks of dead rock (besides another very imperfect
one), entirely submerged; a third, with merely two or three very small
pieces of living reef rising to the surface; and a fourth, namely,
Peros Banhos (Plate I., Figure 9), with a portion nine miles in length
dead and submerged. As by our theory this area has subsided, and as
there is nothing improbable in the death, either from changes in the
state of the surrounding sea or from the subsidence being great or
sudden, of the corals on the whole, or on portions of some of the
atolls, the case of the Chagos group presents no difficulty. So far
indeed are any of the above-mentioned cases of submerged reefs from
being inexplicable, that their occurrence might have been anticipated
on our theory, and as fresh atolls are supposed to be in progressive
formation by the subsidence of encircling barrier-reefs, a weighty
objection, namely that the number of atolls must be increasing
infinitely, might even have been raised, if proofs of the occasional
destruction and loss of atolls could not have been adduced.
_The disseverment of the larger Maldiva atolls_.—The apparent
progressive disseverment in the Maldiva Archipelago of large atolls
into smaller ones, is, in many respects, an important consideration,
and requires an explanation. The graduated series which marks, as I
believe, this process, can be observed only in the northern half of the
group, where the atolls have exceedingly imperfect margins, consisting
of detached basin-formed reefs. The currents of the sea flow across
these atolls, as I am informed by Captain Moresby, with considerable
force, and drift the sediment from side to side during the monsoons,
transporting much of it seaward; yet the currents sweep with greater
force round their flanks. It is historically known that these atolls
have long existed in their present state; and we can believe, that even
during a very slow subsidence they might thus remain, the central
expanse being kept at nearly its original depth by the accumulation of
sediment. But in the action of such nicely balanced forces during a
progressive subsidence (like that, to which by our theory this
archipelago has been subjected), it would be strange if the currents of
the sea should never make a direct passage across some one of the
atolls, through the many wide breaches in their margins. If this were
once effected, a deep-water channel would soon be formed by the removal
of the finer sediment, and the check to its further accumulation; and
the sides of the channel would be worn into a slope like that on the
outer coasts, which are exposed to the same force of the currents. In
fact, a channel precisely like that bifurcating one which divides
Mahlos Mahdoo (Plate II., Figure 4.), would almost necessarily be
formed. The scattered reefs situated near the borders of the new
ocean-channel, from being favourably placed for the growth of coral,
would, by their extension, tend to produce fresh margins to the
dissevered portions; such a tendency is very evident (as may be seen in
the large published chart) in the elongated reefs on the borders of the
two channels intersecting Mahlos Mahdoo. Such channels would become
deeper with continued subsidence, and probably from the reefs not
growing up perpendicularly, somewhat broader. In this case, and more
especially if the channels had been formed originally of considerable
breadth, the dissevered portions would become perfect and distinct
atolls, like Ari and Ross atolls (Plate II., Figure 6), or like the two
Nillandoo atolls, which must be considered as distinct, although
related in form and position, and separated from each other by
channels, which though deep have been sounded. Further subsidence would
render such channels unfathomable, and the dissevered portions would
then resemble Phaleedoo and Moluque atolls, or Mahlos Mahdoo and
Horsburgh atolls (Plate II., Figure 4), which are related to each other
in no respect except in proximity and position. Hence, on the theory of
subsidence, the disseverment of large atolls, which have imperfect
margins (for otherwise their disseverment would be scarcely possible),
and which are exposed to strong currents, is far from being an
improbable event; and the several stages, from close relation to entire
isolation in the atolls of the Maldiva Archipelago, are readily
explicable.
We might go even further, and assert as not improbable, that the first
formation of the Maldiva Archipelago was due to a barrier-reef, of
nearly the same dimensions with that of New Caledonia (Plate II.,
Figure 5), for if, in imagination, we complete the subsidence of that
great island, we might anticipate from the present broken condition of
the northern portion of the reef, and from the almost entire absence of
reefs on the eastern coast, that the barrier-reef after repeated
subsidences, would become during its upward growth separated into
distinct portions; and these portions would tend to assume an
atoll-like structure, from the coral growing with vigour round their
entire circumferences, when freely exposed to an open sea. As we have
some large islands partly submerged with barrier-reefs marking their
former limits, such as New Caledonia, so our theory makes it probable
that there should be other large islands wholly submerged; and these,
we may now infer, would be surmounted, not by one enormous atoll, but
by several large elongated ones, like the atolls in the Maldiva group;
and these again, during long periods of subsidence, would sometimes
become dissevered into smaller atolls. I may add, that both in the
Marshall and Caroline Archipelagoes, there are atolls standing close
together, which have an evident relationship in form: we may suppose,
in such cases, either that two or more encircled islands originally
stood close together, and afforded bases for two or more atolls, or
that one atoll has been dissevered. From the position, as well as form,
of three atolls in the Caroline Archipelago (the Namourrek and Elato
group), which are placed in an irregular circle, I am strongly tempted
to believe that they have originated by the process of
disseverment.[16]
[16] The same remark is, perhaps, applicable to the islands of Ollap,
Fanadik, and Tamatam in the Caroline Archipelago, of which charts are
given in the atlas of Duperrey’s voyage: a line drawn through the
linear reefs and lagoons of these three islands forms a semicircle.
Consult also, the atlas of Lutké’s voyage; and for the Marshall group
that of Kotzebue; for the Gilbert group consult the atlas of
Duperrey’s voyage. Most of the points here referred to may, however,
be seen in Krusenstern’s general Atlas of the Pacific.
_Irregularly formed Atolls_.—In the Marshall group, Musquillo atoll
consists of two loops united in one point; and Menchikoff atoll is
formed of three loops, two of which (as may be seen in Figure 3, Plate
II.) are connected by a mere ribbon-shaped reef, and the three together
are sixty miles in length. In the Gilbert group some of the atolls have
narrow strips of reef, like spurs, projecting from them. There occur
also in parts of the open sea, a few linear and straight reefs,
standing by themselves; and likewise some few reefs in the form of
crescents, with their extremities more or less curled inwards. Now, the
upward growth of a barrier-reef which fronted only one side of an
island, or one side of an elongated island with its extremities (of
which cases exist), would produce after the complete subsidence of the
land, mere strips or crescent or hook-formed reefs: if the island thus
partially fronted became divided during subsidence into two or more
islands, these islands would be united together by linear reefs; and
from the further growth of the coral along their shores together with
subsidence, reefs of various forms might ultimately be produced, either
atolls united together by linear reefs, or atolls with spurs projecting
from them. Some, however, of the more simple forms above specified,
might, as we have seen, be equally well produced by the coral perishing
during subsidence on part of the circumference of an atoll, whilst on
the other parts it continued to grow up till it reached the surface.
_The Great Chagos Bank_.—I have already shown that the submerged
condition of the Great Chagos Bank (Plate II., Figure 1, with its
section Figure 2), and of some other banks in the Chagos group, may in
all probability be attributed to the coral having perished before or
during the movements of subsidence, to which this whole area by our
theory has been subjected. The external rim or upper ledge (shaded in
the chart), consists of dead coral-rock thinly covered with sand; it
lies at an average depth of between five and eight fathoms, and
perfectly resembles in form the annular reef of an atoll. The banks of
the second level, the boundaries of which are marked by dotted lines in
the chart, lie from about fifteen to twenty fathoms beneath the
surface; they are several miles broad, and terminate in a very steep
slope round the central expanse. This central expanse I have already
described, as consisting of a level muddy flat between thirty and forty
fathoms deep. The banks of the second level, might at first sight be
thought analogous to the internal step-like ledge of coral-rock which
borders the lagoons of some atolls, but their much greater width, and
their being formed of sand, are points of essential difference. On the
eastern side of the atoll some of the banks are linear and parallel,
resembling islets in a great river, and pointed directly towards a
great breach on the opposite side of the atoll; these are best seen in
the large published chart. I inferred from this circumstance, that
strong currents sometimes set directly across this vast bank; and I
have since heard from Captain Moresby that this is the case. I
observed, also, that the channels or breaches through the rim, were all
of the same depth as the central lagoon-like space into which they
lead; whereas the channels into the other atolls of the Chagos group,
and as I believe into most other large atolls, are not nearly as deep
as their lagoons: for instance at Peros Banhos, the channels are only
of the same depth, namely between ten and twenty fathoms, as the bottom
of the lagoon for a space about a mile and a half in width round its
shores, whilst the central expanse of the lagoon is from thirty-five to
forty fathoms deep. Now, if an atoll during a gradual subsidence once
became entirely submerged, like the Great Chagos Bank, and therefore no
longer exposed to the surf, very little sediment could be formed from
it; and consequently the channels leading into the lagoon from not
being filled up with drifted sand and coral detritus, would continue
increasing in depth, as the whole sank down. In this case, we might
expect that the currents of the open sea, instead of any longer
sweeping round the submarine flanks, would flow directly through the
breaches across the lagoon, removing in their course the finer
sediment, and preventing its further accumulation. We should then have
the submerged reef forming an external and upper rim of rock, and
beneath this portion of the sandy bottom of the old lagoon, intersected
by deep-water channels or breaches, and thus formed into separate
marginal banks; and these would be cut off by steep slopes, overhanging
the central space, worn down by the passage of the oceanic currents.
By these means, I have scarcely any doubt that the Great Chagos Bank
has originated,—a structure which at first appeared to me far more
anomalous than any I had met with. The process of formation is nearly
the same with that, by which Mahlos Mahdoo had been trisected; but in
the Chagos Bank the channels of the oceanic currents entering at
several different quarters, have united in a central space.
This great atoll-formed bank appears to be in an early stage of
disseverment; should the work of subsidence go on, from the submerged
and dead condition of the whole reef, and the imperfection of the
south-east quarter a mere wreck would probably be left. The Pitt’s
Bank, situated not far southward, appears to be precisely in this
state; it consists of a moderately level, oblong bank of sand, lying
from 10 to 20 fathoms beneath the surface, with two sides protected by
a narrow ledge of rock which is submerged between 5 and 8 fathoms. A
little further south, at about the same distance as the southern rim of
the Great Chagos Bank is from the northern rim, there are two other
small banks with from 10 to 20 fathoms on them; and not far eastward
soundings were struck on a sandy bottom, with between 110 and 145
fathoms. The northern portion with its ledge-like margin, closely
resembles any one segment of the Great Chagos Bank, between two of the
deep-water channels, and the scattered banks, southward appear to be
the last wrecks of less perfect portions.
I have examined with care the charts of the Indian and Pacific Oceans,
and have now brought before the reader all the examples, which I have
met with, of reefs differing from the type of the class to which they
belong; and I think it has been satisfactorily shown, that they are all
included in our theory, modified by occasional accidents which might
have been anticipated as probable. In this course we have seen, that in
the lapse of ages encircling barrier-reefs are occasionally converted
into atolls, the name of atoll being properly applicable, at the moment
when the last pinnacle of encircled land sinks beneath the surface of
the sea. We have, also, seen that large atolls during the progressive
subsidence of the areas in which they stand, sometimes become
dissevered into smaller ones; at other times, the reef-building
polypifers having entirely perished, atolls are converted into
atoll-formed banks of dead rock; and these again through further
subsidence and the accumulation of sediment modified by the force of
the oceanic currents, pass into level banks with scarcely any
distinguishing character. Thus may the history of an atoll be followed
from its first origin, through the occasional accidents of its
existence, to its destruction and final obliteration.
_Objections to the theory of the formation of Atolls and
Barrier-reefs_.—The vast amount of subsidence, both horizontally or in
area, and vertically or in depth, necessary to have submerged every
mountain, even the highest, throughout the immense spaces of ocean
interspersed with atolls, will probably strike most people as a
formidable objection to my theory. But as continents, as large as the
spaces supposed to have subsided, have been raised above the level of
the sea,—as whole regions are now rising, for instance, in Scandinavia
and South America,—and as no reason can be assigned, why subsidences
should not have occurred in some parts of the earth’s crust on as great
a scale both in extent and amount as those of elevation, objections of
this nature strike me as of little force. The remarkable point is that
movements to such an extent should have taken place within a period,
during which the polypifers have continued adding matter on and above
the same reefs. Another and less obvious objection to the theory will
perhaps be advanced from the circumstance, of the lagoons within atolls
and within barrier-reefs never having become in any one instance during
prolonged subsidences of a greater depth than sixty fathoms, and seldom
more than forty fathoms; but we already admit, if the theory be worth
considering, that the rate of subsidence has not exceeded that of the
upward growth of the coral on the exterior margin; we are, therefore,
only further required to admit, that the subsidence has not exceeded in
rate the filling up of the interior spaces by the growth of the corals
living there, and by the accumulation of sediment. As this filling up
must take place very slowly within barrier-reefs lying far from the
land, and within atolls which are of large dimensions and which have
open lagoons with very few reefs, we are led to conclude that the
subsidence thus counter-balanced, must have been slow in an
extraordinary degree; a conclusion which accords with our only means,
namely, with what is known of the rate and manner of recent elevatory
movements, of judging by analogy what is the probable rate of
subsidence.
In this chapter it has, I think, been shown, that the theory of
subsidence, which we were compelled to receive from the necessity of
giving to the corals, in certain large areas, foundations at the
requisite depth, explains both the normal structure and the less
regular forms of those two great classes of reefs, which have justly
excited the astonishment of all persons who have sailed through the
Pacific and Indian Oceans. But further to test the truth of the theory,
a crowd of questions will occur to the reader: Do the different kinds
of reefs, which have been produced by the same kind of movement,
generally lie within the same areas? What is their relation of form and
position,—for instance, do adjoining groups of atolls, and the separate
atolls in these groups, bear the same relation to each other which
islands do in common archipelagoes? Have we reason to believe, that
where there are fringing-reefs, there has not lately been subsidence;
or, for it is almost our only way of ascertaining this point, are there
frequently proofs of recent elevation? Can we by this means account for
the presence of certain classes of reefs in some large areas, and their
entire absence in others? Do the areas which have subsided, as
indicated by the presence of atolls and barrier-reefs, and the areas
which have remained stationary or have been upraised, as shown by
fringing-reefs, bear any determinate relation to each other; and are
the dimensions of these areas such as harmonise with the greatness of
the subterranean changes, which, it must be supposed, have lately taken
place beneath them? Is there any connection between the movements thus
indicated, and recent volcanic action? All these questions ought to
receive answers in accordance with the theory; and if this can be
satisfactorily shown, not only is the theory confirmed, but as
deductions, the answers are in themselves important. Under this latter
point of view, these questions will be chiefly considered in the
following chapter.[17]
[17] I may take this opportunity of briefly considering the
appearances, which would probably be presented by a vertical and deep
section across a coral formation (referring chiefly to an atoll),
formed by the upward growth of coral during successive subsidences.
This is a subject worthy of attention, as a means of comparison with
ancient coral-strata. The circumferential parts would consist of
massive species, in a vertical position, with their interstices filled
up with detritus; but this would be the part most subject to
subsequent denudation and removal. It is useless to speculate how
large a portion of the exterior annular reef would consist of upright
coral, and how much of fragmentary rock, for this would depend on many
contingencies,—such as on the rate of subsidence, occasionally
allowing a fresh growth of coral to cover the whole surface, and on
the breakers having force sufficient to throw fragments over this same
space. The conglomerate which composes the base of the islets, would
(if not removed by denudation together with the exterior reef on which
it rests) be conspicuous from the size of the fragments,—the different
degrees in which they have been rounded,—the presence of fragments of
conglomerate torn up, rounded, and recemented,—and from the oblique
stratification. The corals which lived in the lagoon-reefs at each
successive level, would be preserved upright, and they would consist
of many kinds, generally much branched. In this part, however, a very
large proportion of the rock (and in some cases nearly all of it)
would be formed of sedimentary matter, either in an excessively fine,
or in a moderately coarse state, and with the particles almost blended
together. The conglomerate which was formed of rounded pieces of the
branched corals, on the shores of the lagoon, would differ from that
formed on the islets and derived from the outer coast; yet both might
have accumulated very near each other. I have seen a conglomerate
limestone from Devonshire like a conglomerate now forming on the
shores of the Maldiva atolls. The stratification taken as a whole,
would be horizontal; but the conglomerate beds resting on the exterior
reef, and the beds of sandstone on the shores of the lagoon (and no
doubt on the external flanks) would probably be divided (as at Keeling
atoll and at Mauritius) by numerous layers dipping at considerable
angles in different directions. The calcareous sandstone and
coral-rock would almost necessarily contain innumerable shells,
echini, and the bones of fish, turtle, and perhaps of birds; possibly,
also, the bones of small saurians, as these animals find their way to
the islands far remote from any continent. The large shells of some
species of Tridacna would be found vertically imbedded in the solid
rock, in the position in which they lived. We might expect also to
find a mixture of the remains of pelagic and littoral animals in the
strata formed in the lagoon, for pumice and the seeds of plants are
floated from distant countries into the lagoons of many atolls: on the
outer coast of Keeling atoll, near the mouth of the lagoon, the case
of a pelagic Pteropodous animal was brought up on the arming of the
sounding lead. All the loose blocks of coral on Keeling atoll were
burrowed by vermiform animals; and as every cavity, no doubt,
ultimately becomes filled with spathose limestone, slabs of the rock
taken from a considerable depth, would, if polished, probably exhibit
the excavations of such burrowing animals. The conglomerate and
fine-grained beds of coral-rock would be hard, sonorous, white and
composed of nearly pure calcareous matter; in some few parts, judging
from the specimens at Keeling atoll, they would probably contain a
small quantity of iron. Floating pumice and scoriae, and occasionally
stones transported in the root of trees (see my “Journal of
Researches,” page 549) appear the only sources, through which foreign
matter is brought to coral-formations standing in the open ocean. The
area over which sediment is transported from coral-reefs must be
considerable: Captain Moresby informs me that during the change of
monsoons the sea is discoloured to a considerable distance off the
Maldiva and Chagos atolls. The sediment of fringing and barrier
coral-reefs must be mingled with the mud, which is brought down from
the land, and is transported seaward through the breaches, which occur
in front of almost every valley. If the atolls of the larger
archipelagoes were upraised, the bed of the ocean being converted into
land, they would form flat-topped mountains, varying in diameter from
a few miles (the smallest atolls being worn away) to sixty miles; and
from being horizontally stratified and of similar composition, they
would, as Mr. Lyell has remarked, falsely appear as if they had
originally been united into one vast continuous mass. Such great
strata of coral-rock would rarely be associated with erupted volcanic
matter, for this could only take place, as may be inferred from what
follows in the next chapter, when the area, in which they were
situated, commenced to rise, or at least ceased to subside. During the
enormous period necessary to effect an elevation of the kind just
alluded to, the surface would necessarily be denuded to a great
thickness; hence it is highly improbable that any fringing-reef, or
even any barrier-reef, at least of those encircling small islands,
would be preserved. From this same cause, the strata which were formed
within the lagoons of atolls and lagoon-channels of barrier-reefs, and
which must consist in a large part of sedimentary matter, would more
often be preserved to future ages, than the exterior solid reef,
composed of massive corals in an upright position; although it is on
this exterior part that the present existence and further growth of
atolls and barrier-reefs entirely depend.
CHAPTER VI.
ON THE DISTRIBUTION OF CORAL-REEFS WITH REFERENCE TO THE THEORY OF
THEIR FORMATION.
Description of the coloured map.—Proximity of atolls and
barrier-reefs.— Relation in form and position of atolls with ordinary
islands.—Direct evidence of subsidence difficult to be detected.—Proofs
of recent elevation where fringing-reefs occur.—Oscillations of
level.—Absence of active volcanoes in the areas of
subsidence.—Immensity of the areas which have been elevated and have
subsided.—Their relation to the present distribution of the land.—Areas
of subsidence elongated, their intersection and alternation with those
of elevation.—Amount and slow rate of the subsidence.—Recapitulation.
It will be convenient to give here a short account of the appended map
(Plate III.) [Inasmuch as the coloured map would have proved too costly
to be given in this series, the indications of colour have been
replaced by numbers referring to the dotted groups of reefs, etc. The
author’s original wording, however, is retained in full, as it will be
easy to refer to the map by the numbers, and thus the flow of the
narrative is undisturbed.]: a fuller one, with the data for colouring
each spot, is reserved for the Appendix; and every place there referred
to may be found in the Index. A larger chart would have been desirable;
but, small as the adjoined one is, it is the result of many months’
labour. I have consulted, as far as I was able, every original voyage
and map; and the colours were first laid down on charts on a larger
scale. The same blue colour, with merely a difference in the depth of
tint, is used for atolls or lagoon-islands, and barrier-reefs, for we
have seen, that as far as the actual coral-formation is concerned, they
have no distinguishing character. Fringing-reefs have been coloured
red, for between them on the one hand, and barrier-reefs and atolls on
the other, there is an important distinction with respect to the depth
beneath the surface, at which we are compelled to believe their
foundations lie. The two distinct colours, therefore, mark two great
types of structure.
The _dark blue colour_ [represented by (3) in our plate] represents
atolls and submerged annular reefs, with deep water in their centres. I
have coloured as atolls, a few low and small coral-islands, without
lagoons; but this has been done only when it clearly appeared that they
originally contained lagoons, since filled up with sediment: when there
were not good grounds for this belief, they have been left uncoloured.
The _pale blue colour_ [represented by (2)] represents barrier-reefs.
The most obvious character of reefs of this class is the broad and
deep-water moat within the reef: but this, like the lagoons of small
atolls, is liable to become filled up with detritus and with reefs of
delicately branched corals: when, therefore, a reef round the entire
circumference of an island extends very far into a profoundly deep sea,
so that it can hardly be confounded with a fringing-reef which must
rest on a foundation of rock within a small depth, it has been coloured
pale blue, although it does not include a deep-water moat: but this has
only been done rarely, and each case is distinctly mentioned in the
Appendix.
The _red colour_ (4) represents reefs fringing the land quite closely
where the sea is deep, and where the bottom is gently inclined
extending to a moderate distance from it, but not having a deep-water
moat or lagoon-like space parallel to the shore. It must be remembered
that fringing-reefs are frequently _breached_ in front of rivers and
valleys by deepish channels, where mud has been deposited. A space of
thirty miles in width has been coloured round or in front of the reefs
of each class, in order that the colours might be conspicuous on the
appended map, which is reduced to so small a scale.
The _vermillion spots_, and streaks (1) represent volcanoes now in
action, or historically known to have been so. They are chiefly laid
down from Von Buch’s work on the Canary Islands; and my reasons for
making a few alterations are given in the note below.[1]
[1] I have also made considerable use of the geological part of
Berghaus’ “Physical Atlas.” Beginning at the eastern side of the
Pacific, I have added to the number of the volcanoes in the southern
part of the Cordillera, and have coloured Juan Fernandez according to
observations collected during the voyage of the _Beagle_ (“Geological
Transactions,” volume v., page 601.) I have added a volcano to
Albemarle Island, one of the Galapagos Archipelago (the author’s
“Journal of Researches,” page 457). In the Sandwich group there are no
active volcanoes, except at Hawaii; but the Rev. W. Ellis informs me,
there are streams of lava apparently modern on Maui, having a very
recent appearance, which can be traced to the craters whence they
flowed. The same gentleman informs me, that there is no reason to
believe that any active volcano exists in the Society Archipelago; nor
are there any known in the Samoa or Navigator group, although some of
the streams of lava and craters there appear recent. In the Friendly
group, the Rev. J. Williams says (“Narrative of Missionary
Enterprise,” page 29) that Toofoa and Proby Islands are active
volcanoes. I infer from Hamilton’s “Voyage in the ‘Pandora’” (Page
95), that Proby Island is synonymous with Onouafou, but I have not
ventured to colour it. There can be no doubt respecting Toofoa, and
Captain Edwards (Von Buch, page 386) found the lava of recent eruption
at Amargura still smoking. Berghaus marks four active volcanoes
actually within the Friendly group; but I do not know on what
authority: I may mention that Maurelle describes Latte as having a
burnt-up appearance: I have marked only Toofoa and Amargura. South of
the New Hebrides lies Matthews Rock, which is drawn and described as
an active crater in the “Voyage of the _Astrolabe_.” Between it and
the volcano on the eastern side of New Zealand, lies Brimstone Island,
which from the high temperature of the water in the crater, may be
ranked as active (Berghaus “Vorbemerk,” II Lief. S. 56). Malte Brun,
volume xii., page 231, says that there is a volcano near port St.
Vincent in New Caledonia. I believe this to be an error, arising from
a smoke seen on the _opposite_ coast by Cook (“Second Voyage,” volume
ii., page 23) which smoke went out at night. The Mariana Islands,
especially the northern ones, contain many craters (see Freycinet’s
“Hydrog. Descript.”) which are not active. Von Buch, however, states
(page 462) on the authority of La Peyrouse, that there are no less
than seven volcanoes between these islands and Japan. Gemelli Creri
(Churchill’s “Collect.” volume iv., page 458), says there are two
active volcanoes in latitude 23° 30′, and in latitude 24°: but I have
not coloured them. From the statements in Beechey’s “Voyage” (page
518, 4to edition) I have coloured one in the northern part of the
Bonin group. M. S. Julien has clearly made out from Chinese
manuscripts not very ancient (“Comptes Rendus,” 1840, page 832), that
there are two active volcanoes on the eastern side of Formosa. In
Torres Straits, on Cap Island (9° 48′ S., 142° 39′ E.) a volcano was
seen burning with great violence in 1793 by Captain Bampton (see
Introduction to Flinders’ “Voyage,” page 41). Mr. M’Clelland (Report
of Committee for investigating Coal in India, page 39) has shown that
the volcanic band passing through Barren Island must be extended
northwards. It appears by an old chart, that Cheduba was once an
active volcano (see also “Silliman’s North American Journal”, volume
xxxviii., page 385). In Berghaus’ “Physical Atlas,” 1840, No. 7 of
Geological Part, a volcano on the coast of Pondicherry is said to have
burst forth in 1757. Ordinaire (“Hist. Nat. des Volcans,” page 218)
says that there is one at the mouth of the Persian Gulf, but I have
not coloured it, as he gives no particulars. A volcano in Amsterdam,
or St. Paul’s, in the southern part of the Indian Ocean, has been seen
(“Naut. Mag.” 1838, page 842) in action. Dr. J. Allan, of Forres,
informs me in a letter, that when he was at Joanna, he saw at night
flames apparently volcanic, issuing from the chief Comoro Island, and
that the Arabs assured him that they were volcanic, adding that the
volcano burned more during the wet season. I have marked this as a
volcano, though with some hesitation, on account of the possibility of
the flame arising from gaseous sources.
The _uncoloured coasts_ consist, first and chiefly, of those, where
there are no coral-reefs, or such small portions as to be quite
insignificant. Secondly, of those coasts where there are reefs, but
where the sea is very shallow, for in this case the reefs generally lie
far from the land, and become very irregular, in their forms: where
they have not become irregular, they have been coloured. thirdly, if I
had the means of ascertaining the fact, I should not colour a reef
merely coating the edges of a submarine crater, or of a level submerged
bank; for such superficial formations differ essentially, even when not
in external appearance, from reefs whose foundations as well as
superficies have been wholly formed by the growth of coral. Fourthly,
in the Red Sea, and within some parts of the East Indian Archipelago
(if the imperfect charts of the latter can be trusted), there are many
scattered reefs, of small size, represented in the chart by mere dots,
which rise out of deep water: these cannot be arranged under either of
the three classes: in the Red Sea, however, some of these little reefs,
from their position, seem once to have formed parts of a continuous
barrier. There exist, also, scattered in the open ocean, some linear
and irregularly formed strips of coral-reef, which, as shown in the
last chapter, are probably allied in their origin to atolls; but as
they do not belong to that class, they have not been coloured; they are
very few in number and of insignificant dimensions. Lastly, some reefs
are left uncoloured from the want of information respecting them, and
some because they are of an intermediate structure between the barrier
and fringing classes. The value of the map is lessened, in proportion
to the number of reefs which I have been obliged to leave uncoloured,
although, in a theoretical point of view, few of them present any great
difficulty: but their number is not very great, as will be found by
comparing the map with the statements in the Appendix. I have
experienced more difficulty in colouring fringing-reefs than in
colouring barrier-reefs, as the former, from their much less
dimensions, have less attracted the attention of navigators. As I have
had to seek my information from all kinds of sources, and often from
indirect ones, I do not venture to hope that the map is free from many
errors. Nevertheless, I trust it will give an approximately correct
view of the general distribution of the coral-reefs over the whole
world (with the exception of some fringing-reefs on the coast of
Brazil, not included within the limits of the map), and of their
arrangement into the three great classes, which, though necessarily
very imperfect from the nature of the objects classified, have been
adopted by most voyagers. I may further remark, that the dark blue
colour represents land entirely composed of coral-rock; the pale blue,
land with a wide and thick border of coral-rock; and the red, a mere
narrow fringe of coral-rock.
Looking now at the map under the theoretical point of view indicated in
the last chapter, the two blue tints signify that the foundations of
the reefs thus coloured have subsided to a considerable amount, at a
slower rate than that of the upward growth of the corals, and that
probably in many cases they are still subsiding. The red signifies that
the shores which support fringing-reefs have not subsided (at least to
any considerable amount, for the effects of a subsidence on a small
scale would in no case be distinguishable); but that they have remained
nearly stationary since the period when they first became fringed by
reefs; or that they are now rising or have been upraised, with new
lines of reefs successively formed on them: these latter alternatives
are obviously implied, as newly formed lines of shore, after elevations
of the land, would be in the same state with respect to the growth of
fringing-reefs, as stationary coasts. If during the prolonged
subsidence of a shore, coral-reefs grew for the first time on it, or if
an old barrier-reef were destroyed and submerged, and new reefs became
attached to the land, these would necessarily at first belong to the
fringing class, and, therefore, be coloured red, although the coast was
sinking: but I have no reason to believe, that from this source of
error, any coast has been coloured wrongly with respect to movement
indicated. Well characterised atolls and encircling barrier-reefs,
where several occur in a group, or a single barrier-reef if of large
dimensions, leave scarcely any doubt on the mind respecting the
movement by which they have been produced; and even a small amount of
subsequent elevation is soon betrayed. The evidence from a single atoll
or a single encircling barrier-reef, must be received with some
caution, for the former may possibly be based upon a submerged crater
or bank, and the latter on a submerged margin of sediment, or of
worn-down rock. From these remarks we may with greater certainty infer
that the spaces, especially the larger ones, tinted blue in the map,
have subsided, than that the red spaces have remained stationary, or
have been upraised.
_On the grouping of the different classes of reefs_.—Having made these
preliminary remarks, I will consider first how far the grouping of the
different kinds of coral-islands and reefs is corroborative of the
truth of the theory. A glance at the map shows that the reefs, coloured
blue and red, produced under widely different conditions, are not
indiscriminately mixed together. Atolls and barrier-reefs, on the other
hand, as may be seen by the two blue tints, generally lie near each
other; and this would be the natural result of both having been
produced during the subsidence of the areas in which they stand. Thus,
the largest group of encircled islands is that of the Society
Archipelago; and these islands are surrounded by atolls, and only
separated by a narrow space from the large group of Low atolls. In the
midst of the Caroline atolls, there are three fine encircled islands.
The northern point of the barrier-reef of New Caledonia seems itself,
as before remarked, to form a complete large atoll. The great
Australian barrier is described as including both atolls and small
encircled islands. Captain King[2] mentions many atoll-formed and
encircling coral-reefs, some of which lie within the barrier, and
others may be said (for instance between latitude 16° and 13°) to form
part of it. Flinders[3] has described an atoll-formed reef in latitude
10°, seven miles long and from one to three broad, resembling a boot in
shape, with apparently very deep water within. Eight miles westward of
this, and forming part of the barrier, lie the Murray Islands, which
are high and are encircled. In the Corallian Sea, between the two great
barriers of Australia and New Caledonia, there are many low islets and
coral-reefs, some of which are annular, or horse-shoe shaped. Observing
the smallness of the scale of the map, the parallels of latitude being
nine hundred miles apart, we see that none of the large groups of reefs
and islands supposed to have been produced by long-continued
subsidence, lie near extensive lines of coast coloured red, which are
supposed to have remained stationary since the growth of their reefs,
or to have been upraised and new lines of reefs formed on them. Where
the red and blue circles do occur near each other, I am able, in
several instances, to show that there have been oscillations of level,
subsidence having preceded the elevation of the red spots; and
elevation having preceded the subsidence of the blue spots: and in this
case the juxtaposition of reefs belonging to the two great types of
structure is little surprising. We may, therefore, conclude that the
proximity in the same areas of the two classes of reefs, which owe
their origin to the subsidence of the earth’s crust, and their
separation from those formed during its stationary or uprising
condition, holds good to the full extent, which might have been
anticipated by our theory.
[2] Sailing directions, appended to volume ii. of his “Surveying
Voyage to Australia.”
[3] “Voyage to Terra Australis,” volume ii. page 336.
As groups of atolls have originated in the upward growth, at each fresh
sinking of the land, of those reefs which primarily fringed the shores
of one great island, or of several smaller ones; so we might expect
that these rings of coral-rock, like so many rude outline charts, will
still retain some traces of the general form, or at least general
range, of the land, round which they were first modelled. That this is
the case with the atolls in the Southern Pacific as far as their range
is concerned, seems highly probable, when we observe that the three
principal groups are directed in north-west and south-east lines, and
that nearly all the land in the S. Pacific ranges in this same
direction; namely, N. Western Australia, New Caledonia, the northern
half of New Zealand, the New Hebrides, Saloman, Navigator, Society,
Marquesas, and Austral archipelagoes: in the Northern Pacific, the
Caroline atolls abut against the north-west line of the Marshall
atolls, much in the same manner as the east and west line of islands
from Ceram to New Britain do on New Ireland: in the Indian Ocean the
Laccadive and Maldiva atolls extend nearly parallel to the western and
mountainous coast of India. In most respects, there is a perfect
resemblance with ordinary islands in the grouping of atolls and in
their form: thus the outline of all the larger groups is elongated; and
the greater number of the individual atolls are elongated in the same
direction with the group, in which they stand. The Chagos group is less
elongated than is usual with other groups, and the individual atolls in
it are likewise but little elongated; this is strikingly seen by
comparing them with the neighbouring Maldiva atolls. In the Marshall
and Maldiva archipelagoes, the atolls are ranged in two parallel lines,
like the mountains in a great double mountain-chain. Some of the
atolls, in the larger archipelagoes, stand so near to each other, and
have such an evident relationship in form, that they compose little
sub-groups: in the Caroline Archipelago, one such sub-group consists of
Pouynipète, a lofty island encircled by a barrier-reef, and separated
by a channel only four miles and a half wide from Andeema atoll, with a
second atoll a little further off. In all these respects an examination
of a series of charts will show how perfectly groups of atolls resemble
groups of common islands.
_On the direct evidence of the blue spaces in the map having subsided
during the upward growth of the reefs so coloured, and of the red
spaces having remained stationary, or having been upraised._—With
respect to subsidence, I have shown in the last chapter, that we cannot
expect to obtain in countries inhabited only by semi-civilised races,
demonstrative proofs of a movement, which invariably tends to conceal
its own evidence. But on the coral-islands supposed to have been
produced by subsidence, we have proofs of changes in their external
appearance—of a round of decay and renovation—of the last vestiges of
land on some—of its first commencement on others: we hear of storms
desolating them to the astonishment of their inhabitants: we know by
the great fissures with which some of them are traversed, and by the
earthquakes felt under others, that subterranean disturbances of some
kind are in progress. These facts, if not directly connected with
subsidence, as I believe they are, at least show how difficult it would
be to discover proofs of such movement by ordinary means. At Keeling
atoll, however, I have described some appearances, which seem directly
to show that subsidence did take place there during the late
earthquakes. Vanikoro, according to Chevalier Dillon,[4] is often
violently shaken by earthquakes, and there, the unusual depth of the
channel between the shore and the reef,—the almost entire absence of
islets on the reef,— its wall-like structure on the inner side, and the
small quantity of low alluvial land at the foot of the mountains, all
seem to show that this island has not remained long at its present
level, with the lagoon-channel subjected to the accumulation of
sediment, and the reef to the wear and tear of the breakers. At the
Society Archipelago, on the other hand, where a slight tremor is only
rarely felt, the shoaliness of the lagoon-channels round some of the
islands, the number of islets formed on the reefs of others, and the
broad belt of low land at the foot of the mountains, indicate that,
although there must have been great subsidence to have produced the
barrier-reefs, there has since elapsed a long stationary period.[5]
[4] See Captain Dillon’s “Voyage in search of La Peyrouse.” M. Cordier
in his “Report on the Voyage of the ‘Astrolabe’” (page cxi., volume
i.), speaking of Vanikoro, says the shores are surrounded by reefs of
madrepore, _“qu’on assure être de formation tout-a-fait moderne_.” I
have in vain endeavoured to learn some further particulars about this
remarkable passage. I may here add, that according to our theory, the
island of Pouynipète (Plate I., Figure 7), in the Caroline
Archipelago, being encircled by a barrier-reef, must have subsided. In
the “New S. Wales Lit. Advert.” February 1835 (which I have seen
through the favour of Dr. Lloghtsky), there is an account of this
island (subsequently confirmed by Mr. Campbell), in which it is said,
“At the N.E. end, at a place called Tamen, there are ruins of a town,
_now only_ accessible by boats, the waves _reaching to the steps of
the houses_.” Judging from this passage, one would be tempted to
conclude that the island must have subsided, since these houses were
built. I may, also, here append a statement in Malte Brun (volume ix.,
page 775, given without any authority), that the sea gains in an
extraordinary manner on the coast of Cochin China, which lies in front
and near the subsiding coral-reefs in the China Sea: as the coast is
granitic, and not alluvial, it is scarcely possible that the
encroachment of the sea can be owing to the washing away of the land;
and if so, it must be due to subsidence.
[5] Mr. Couthouy states (“Remarks,” page 44) that at Tahiti and Eimeo
the space between the reef and the shore has been nearly filled up by
the extension of those coral-reefs, which within most barrier-reefs
merely fringe the land. From this circumstance, he arrives at the same
conclusion as I have done, that the Society Islands since their
subsidence, have remained stationary during a long period; but he
further believes that they have recently commenced rising, as well as
the whole area of the Low Archipelago. He does not give any detailed
proofs regarding the elevation of the Society Islands, but I shall
refer to this subject in another part of this chapter. Before making
some further comments, I may observe how satisfactory it is to me, to
find Mr. Couthouy affirming, that “having personally examined a large
number of coral-islands, and also residing eight months among the
volcanic class, having shore and partially encircling reefs, I may be
permitted to state that my own observations have impressed a
conviction of the correctness of the theory of Mr. Darwin.”
This gentleman believes, that subsequently to the subsidence by
which the atolls in the Low Archipelago were produced, the whole
area has been elevated to the amount of a few feet; this would
indeed be a remarkable fact; but as far as I am able to judge, the
grounds of his conclusion are not sufficiently strong. He states
that he found in almost every atoll which he visited, the shores of
the lagoon raised from eighteen to thirty inches above the
sea-level, and containing imbedded Tridacnae and corals standing as
they grew; some of the corals were dead in their upper parts, but
below a certain line they continued to flourish. In the lagoons,
also, he frequently met with clusters of Madrepore, with their
extremities standing from one inch to a foot above the surface of
the water. Now, these appearances are exactly what I should have
expected, without any subsequent elevation having taken place; and
I think Mr. Couthouy has not borne in mind the indisputable fact,
that corals, when constantly bathed by the surf, can exist at a
higher level than in quite tranquil water, as in a lagoon. As long,
therefore, as the waves continued at low water to break entirely
over parts of the annular reef of an atoll, submerged to a small
depth, the corals and shells attached on these parts might continue
living at a level above the smooth surface of the lagoon, into
which the waves rolled; but as soon as the outer edge of the reef
grew up to its utmost possible height, or if the reef were very
broad nearly to that height, the force of the breakers would be
checked, and the corals and shells on the inner parts near the
lagoon would occasionally be left dry, and thus be partially or
wholly destroyed. Even in atolls, which have not lately subsided,
if the outer margin of the reef continued to increase in breadth
seaward (each fresh zone of corals rising to the same vertical
height as at Keeling atoll), the line where the waves broke most
heavily would advance outwards, and therefore the corals, which
when living near the margin, were washed by the breaking waves
during the whole of each tide, would cease being so, and would
therefore be left on the backward part of the reef standing exposed
and dead. The case of the madrepores in the lagoons with the tops
of their branches exposed, seems to be an analogous fact, to the
great fields of dead but upright corals in the lagoon of Keeling
atoll; a condition of things which I have endeavoured to show, has
resulted from the lagoon having become more and more enclosed and
choked up with reefs, so that during high winds, the rising of the
tide (as observed by the inhabitants) is checked, and the corals,
which had formerly grown to the greatest possible height, are
occasionally exposed, and thus are killed: and this is a condition
of things, towards which almost every atoll in the intervals of its
subsidence must be tending. Or if we look to the state of an atoll
directly after a subsidence of some fathoms, the waves would roll
heavily over the entire circumference of the reef, and the surface
of the lagoon would, like the ocean, never be quite at rest, and
therefore the corals in the lagoon, from being constantly laved by
the rippling water, might extend their branches to a little greater
height than they could, when the lagoon became enclosed and
protected. Christmas atoll (2° N. latitude) which has a very
shallow lagoon, and differs in several respects from most atolls,
possibly may have been elevated recently; but its highest part
appears (Couthouy, page 46) to be only ten feet above the
sea-level. The facts of a second class, adduced by Mr. Couthouy, in
support of the alleged recent elevation of the Low Archipelago, are
not all (especially those referring to a shelf of rock) quite
intelligible to me; he believes that certain enormous fragments of
rock on the reef, must have been moved into their present position,
when the reef was at a lower level; but here again the force of the
breakers on any inner point of the reef being diminished by its
outward growth without any change in its level, has not, I think,
been borne in mind. We should, also, not overlook the occasional
agency of waves caused by earthquakes and hurricanes. Mr. Couthouy
further argues, that since these great fragments were deposited and
fixed on the reef, they have been elevated; he infers this from the
greatest amount of erosion not being near their bases, where they
are unceasingly washed by the reflux of the tides, but at some
height on their sides, near the line of high-water mark, as shown
in an accompanying diagram. My former remark again applies here,
with this further observation, that as the waves have to roll over
a wide space of reef before they reach the fragments, their force
must be greatly increased with the increasing depth of water as the
tide rises, and therefore I should have expected that the chief
line of present erosion would have coincided with the line of
high-water mark; and if the reef had grown outwards, that there
would have been lines of erosion at greater heights. The
conclusion, to which I am finally led by the interesting
observations of Mr. Couthouy is, that the atolls in the Low
Archipelago have, like the Society Islands, remained at a
stationary level for a long period: and this probably is the
ordinary course of events, subsidence supervening after long
intervals of rest.
Turning now to the red colour; as on our map, the areas which have sunk
slowly downwards to great depths are many and large, we might naturally
have been led to conjecture, that with such great changes of level in
progress, the coasts which have been fringed probably for ages (for we
have no reason to believe that coral-reefs are of short duration),
would not have remained all this time stationary, but would frequently
have undergone movements of elevation. This supposition, we shall
immediately see, holds good to a remarkable extent; and although a
stationary condition of the land can hardly ever be open to proof, from
the evidence being only negative, we are, in some degree, enabled to
ascertain the correctness of the parts coloured red on the map, by the
direct testimony of upraised organic remains of a modern date. Before
going into the details on this head (printed in small type), I may
mention, that when reading a memoir on coral formations by MM. Quoy and
Gaimard[6] I was astonished to find, for I knew that they had crossed
both the Pacific and Indian Oceans, that their descriptions were
applicable only to reefs of the fringing class; but my astonishment
ended satisfactorily, when I discovered that, by a strange chance, all
the islands which these eminent naturalists had visited, though several
in number, namely, the Mauritius, Timor, New Guinea, the Mariana, and
Sandwich Archipelagoes, could be shown by their own statements to have
been elevated within a recent geological era.
[6] “Annales des Sciences Nat.” tom. vi., page 279, etc.
In the eastern half of the Pacific, the _Sandwich Islands_ are all
fringed, and almost every naturalist who has visited them, has remarked
on the abundance of elevated corals and shells, apparently identical
with living species. The Rev. W. Ellis informs me, that he has noticed
round several parts of Hawaii, beds of coral-detritus, about twenty
feet above the level of the sea, and where the coast is low they extend
far inland. Upraised coral-rock forms a considerable part of the
borders of Oahu; and at Elizabeth Island[7] it composes three strata,
each about ten feet thick. Nihau, which forms the northern, as Hawaii
does the southern end of the group (350 miles in length), likewise
seems to consist of coral and volcanic rocks. Mr. Couthouy[8] has
lately described with interesting details, several upraised beaches,
ancient reefs with their surfaces perfectly preserved, and beds of
recent shells and corals, at the islands of Maui, Morokai, Oahu, and
Tauai (or Kauai) in this group. Mr. Pierce, an intelligent resident at
Oahu, is convinced, from changes which have taken place within his
memory, during the last sixteen years, “that the elevation is at
present going forward at a very perceptible rate.” The natives at Kauai
state that the land is there gaining rapidly on the sea, and Mr.
Couthouy has no doubt, from the nature of the strata, that this has
been effected by an elevation of the land.
[7] “Zoology of Captain Beechey’s Voyage,” page 176. See also MM. Quoy
and Gaimard in “Annales de Scien. Nat.” tom. vi.
[8] “Remarks on Coral Formations,” page 51.
In the southern part of the Low Archipelago, Elizabeth Island is
described by Captain Beechey,[9] as being quite flat, and about eighty
feet in height; it is entirely composed of dead corals, forming a
honeycombed, but compact rock. In cases like this, of an island having
exactly the appearance, which the elevation of any one of the smaller
surrounding atolls with a shallow lagoon would present, one is led to
conclude (with little better reason, however, than the improbability of
such small and low fabrics lasting, for an immense period, exposed to
the many destroying agents of nature), that the elevation has taken
place at an epoch not geologically remote. When merely the surface of
an island of ordinary formation is strewed with marine bodies, and that
continuously, or nearly so, from the beach to a certain height, and not
above that height, it is exceedingly improbable that such organic
remains, although they may not have been specially examined, should
belong to any ancient period. It is necessary to bear these remarks in
mind, in considering the evidence of the elevatory movements in the
Pacific and Indian Oceans, as it does not often rest on specific
determinations, and therefore should be received with caution. Six of
the _Cook and Austral Islands_ (S.W. of the Society group), are
fringed; of these, five were described to me by the Rev. J. Williams,
as formed of coral-rock, associated with some basalt in Mangaia), and
the sixth as lofty and basaltic. Mangaia is nearly three hundred feet
high, with a level summit; and according to Mr. S. Wilson[10] it is an
upraised reef; “and there are in the central hollow, formerly the bed
of the lagoon, many scattered patches of coral-rock, some of them
raised to a height of forty feet.” These knolls of coral-rock were
evidently once separate reefs in the lagoon of an atoll. Mr. Martens,
at Sydney, informed me that this island is surrounded by a terrace-like
plain at about the height of a hundred feet, which probably marks a
pause in its elevation. From these facts we may infer, perhaps, that
the Cook and Austral Islands have been upheaved at a period probably
not very remote.
[9] Beechey’s “Voyage in the Pacific,” page 46, 4to edition.
[10] Couthouy’s “Remarks,” page 34.
_Savage Island_ (S.E. of the Friendly group), is about forty feet in
height. Forster[11] describes the plants as _already_ growing out of
the dead, but still upright and spreading trees of coral; and the
younger Forster[12] believes that an ancient lagoon is now represented
by a central plain; here we cannot doubt that the elevatory forces have
recently acted. The same conclusion may be extended, though with
somewhat less certainty, to the islands of the _Friendly Group_, which
have been well described in the second and third voyages of Cook. The
surface of Tongatabou is low and level, but with some parts a hundred
feet high; the whole consists of coral-rock, “which yet shows the
cavities and irregularities worn into it by the action of the
tides.”[13] On Eoua the same appearances were noticed at an elevation
of between two hundred and three hundred feet. Vavao, also, at the
opposite or northern end of the group, consists, according to the Rev.
J. Williams, of coral-rock. Tongatabou, with its northern extensive
reefs, resembles either an upraised atoll with one half originally
imperfect, or one unequally elevated; and Anamouka, an atoll equally
elevated. This latter island contains[14] in its centre a salt-water
lake, about a mile-and-a-half in diameter, without any communication
with the sea, and around it the land rises gradually like a bank; the
highest part is only between twenty and thirty feet; but on this part,
as well as on the rest of the land (which, as Cook observes, rises
above the height of true lagoon-islands), coral-rock, like that on the
beach, was found. In the _Navigator Archipelago_, Mr. Couthouy[15]
found on Manua many and very large fragments of coral at the height of
eighty feet, “on a steep hill-side, rising half a mile inland from a
low sandy plain abounding in marine remains.” The fragments were
embedded in a mixture of decomposed lava and sand. It is not stated
whether they were accompanied by shells, or whether the corals
resembled recent species; as these remains were embedded they possibly
may belong to a remote epoch; but I presume this was not the opinion of
Mr. Couthouy. Earthquakes are very frequent in this archipelago.
[11] “Observations made during Voyage round the World,” page 147.
[12] “Voyage,” volume ii., page 163.
[13] Cook’s “Third Voyage” (4to edition), volume i., page 314.
[14] Ibid., volume i., page 235.
[15] “Remarks on Coral-Formations,” page 50.
Still proceeding westward we come to the _New Hebrides;_ on these
islands, Mr. G. Bennett (author of “Wanderings in New South Wales”),
informs me he found much coral at a great altitude, which he considered
of recent origin. Respecting _Santa Cruz_, and the _Salomon
Archipelago_, I have no information; but at New Ireland, which forms
the northern point of the latter chain, both Labillardière and Lesson
have described large beds of an apparently very modern madreporitic
rock, with the form of the corals little altered. The latter author[16]
states that this formation composes a newer line of coast, modelled
round an ancient one. There only remains to be described in the
Pacific, that curved line of fringed islands, of which the _Marianas_
form the main part. Of these Guam, Rota, Tiniam, Saypan, and some
islets farther north, are described by Quoy and Gaimard,[17] and
Chamisso,[18] as chiefly composed of madreporitic limestone, which
attains a considerable elevation, and is in several cases worn into
successively rising cliffs: the two former naturalists seem to have
compared the corals and shells with the existing ones, and state that
they are of recent species. _Fais_, which lies in the prolonged line of
the Marianas, is the only island in this part of the sea which is
fringed; it is ninety feet high, and consists entirely of madreporitic
rock.[19]
[16] “Voyage de la ‘Coquille’,” Part. Zoolog.
[17] Freycinet’s “Voyage autour du Monde.” See also the
“Hydrographical Memoir,” page 215.
[18] Kotzebue’s “First Voyage.”
[19] Lutké’s “Voyage,” volume ii., page 304.
In the _East Indian Archipelago_, many authors have recorded proofs of
recent elevation. M. Lesson[20] states, that near Port Dory, on the
north coast of New Guinea, the shores are flanked, to the height of 150
feet, by madreporitic strata of modern date. He mentions similar
formations at Waigiou, Amboina, Bourou, Ceram, Sonda, and Timor: at
this latter place, MM. Quoy and Gaimard[21] have likewise described the
primitive rocks, as coated to a considerable height with coral. Some
small islets eastward of Timor are said in Kolff’s “Voyage,”[22] to
resemble small coral islets upraised some feet above the sea. Dr.
Malcolmson informs me that Dr. Hardie found in _Java_ an extensive
formation, containing an abundance of shells, of which the greater part
appear to be of existing species. Dr. Jack[23] has described some
upraised shells and corals, apparently recent, on Pulo Nias off
_Sumatra;_ and Marsden relates in his history of this great island,
that the names of many promontories, show that they were originally
islands. On part of the west coast of _Borneo_ and at the _Sooloo
Islands_, the form of the land, the nature of the soil, and the
water-washed rocks, present appearances[24] (although it is doubtful
whether such vague evidence is worthy of mention), of having recently
been covered by the sea; and the inhabitants of the Sooloo Islands
believe that this has been the case. Mr. Cuming, who has lately
investigated, with so much success, the natural history of the
_Phillippines_, found near Cabagan, in Luzon, about fifty feet above
the level of the R. Cagayan, and seventy miles from its mouth, a large
bed of fossil shells: these, he informs me, are of the same species
with those now existing on the shores of the neighbouring islands. From
the accounts given us by Captain Basil Hall and Captain Beechey[25] of
the lines of inland reefs, and walls of coral-rock worn into caves,
above the present reach of the waves, at the _Loo Choo_ Islands, there
can be little doubt that they have been upraised at no very remote
period.
[20] Partie Zoolog., “Voyage de la ‘Coquille’.”
[21] “Ann. des Scien. Nat.” tom. vi., page 281.
[22] Translated by Windsor Earl, chapters vi., vii.
[23] “Geolog. Transact.” 2nd series, volume i., page 403. On the
Peninsula of Malacca, in front of Pinang, 5° 30′ N., Dr. Ward
collected some shells, which Dr. Malcolmson informs me, although not
compared with existing species, had a recent appearance. Dr. Ward
describes in this neighbourhood (“Trans. Asiat. Soc.” volume xviii.,
part ii., page 166) a single water-worn rock, with a conglomerate of
sea-shells at its base, situated six miles inland, which, according to
the traditions of the natives, was once surrounded by the sea. Captain
Low has also described (Ibid., part i., page 131) mounds of shells
lying two miles inland on this line of coast.
[24] “Notices of the East Indian Arch.” Singapore, 1828, page 6, and
Append., page 43.
[25] Captain B. Hall, “Voyage to Loo Choo,” Append., pages xxi. and
xxv. Captain Beechey’s “Voyage,” page 496.
Dr. Davy[26] describes the northern province of _Ceylon_ as being very
low, and consisting of a limestone with shells and corals of very
recent origin; he adds, that it does not admit of a doubt that the sea
has retired from this district even within the memory of man. There is
also some reason for believing that the western shores of India, north
of Ceylon, have been upraised within the recent period.[27] _Mauritius_
has certainly been upraised within the recent period, as I have stated
in the chapter on fringing-reefs. The northern extremity of
_Madagascar_ is described by Captain Owen[28] as formed of madreporitic
rock, as likewise are the shores and outlying islands along an immense
space of _Eastern Africa_, from a little north of the equator for nine
hundred miles southward. Nothing can be more vague than the expression
“madreporitic rock;” but at the same time it is, I think, scarcely
possible to look at the chart of the linear islets, which rise to a
greater height than can be accounted for by the growth of coral, in
front of the coast, from the equator to 2° S., without feeling
convinced that a line of fringing-reefs has been elevated at a period
so recent, that no great changes have since taken place on the surface
of this part of the globe. Some, also, of the higher islands of
madreporitic rock on this coast, for instance Pemba, have very singular
forms, which seem to show the combined effect of the growth of coral
round submerged banks, and their subsequent upheaval. Dr. Allan informs
me that he never observed any elevated organic remains on the
_Seychelles_, which come under our fringed class.
[26] “Travels in Ceylon,” page 13. This madreporitic formation is
mentioned by M. Cordier in his report to the Institute (May 4th,
1839), on the voyage of the “Chevrette”, as one of immense extent, and
belonging to the latest tertiary period.
[27] Dr. Benza, in his “Journey through the N. Circars” (the “Madras
Lit. and Scient. Journ.” volume v.) has described a formation with
recent fresh-water and marine shells, occurring at the distance of
three or four miles from the present shore. Dr. Benza, in conversation
with me, attributed their position to a rise of the land. Dr.
Malcolmson, however (and there cannot be a higher authority on the
geology of India) informs me that he suspects that these beds may have
been formed by the mere action of the waves and currents accumulating
sediment. From analogy I should much incline to Dr. Benza’s opinion.
[28] Owen’s “Africa,” volume ii., page 37, for Madagascar; and for S.
Africa, volume i., pages 412 and 426. Lieutenant Boteler’s narrative
contains fuller particulars regarding the coral-rock, volume i., page
174, and volume ii., pages 41 and 54. See also Ruschenberger’s “Voyage
round the World,” volume i., page 60.
The nature of the formations round the shores of the _Red Sea_, as
described by several authors, shows that the whole of this large area
has been elevated within a very recent tertiary epoch. A part of this
space in the appended map, is coloured blue, indicating the presence of
barrier-reefs: on which circumstance I shall presently make some
remarks. Rüppell[29] states that the tertiary formation, of which he
has examined the organic remains, forms a fringe along the shores with
a uniform height of from thirty and forty feet from the mouth of the
Gulf of Suez to about latitude 26°; but that south of 26°, the beds
attain only the height of from twelve to fifteen feet. This, however,
can hardly be quite accurate; although possibly there may be a decrease
in the elevation of the shores in the middle parts of the Red Sea, for
Dr. Malcolmson (as he informs me) collected from the cliffs of Camaran
Island (latitude 15° 30′ S.) shells and corals, apparently recent, at a
height between thirty and forty feet; and Mr. Salt (“Travels in
Abyssinia”) describes a similar formation a little southward on the
opposite shore at Amphila. Moreover, near the mouth of the Gulf of
Suez, although on the coast opposite to that on which Dr. Rüppell says
that the modern beds attain a height of only thirty to forty feet, Mr.
Burton[30] found a deposit replete with existing species of shells, at
the height of 200 feet. In an admirable series of drawings by Captain
Moresby, I could see how continuously the cliff-bounded low plains of
this formation extended with a nearly equable height, both on the
eastern and western shores. The southern coast of Arabia seems to have
been subjected to the same elevatory movement, for Dr. Malcolmson found
at Sahar low cliffs containing shells and corals, apparently of recent
species.
[29] Rüppell, “Reise in Abyssinien,” Band i., s. 141.
[30] Lyell’s “Principles of Geology,” 5th edition, volume iv., page
25.
The _Persian Gulf_ abounds with coral-reefs; but as it is difficult to
distinguish them from sand-banks in this shallow sea, I have coloured
only some near the mouth; towards the head of the gulf Mr.
Ainsworth[31] says that the land is worn into terraces, and that the
beds contain organic remains of existing forms. The _West Indian
Archipelago_ of “fringed” islands, alone remains to be mentioned;
evidence of an elevation within a late tertiary epoch of nearly the
whole of this great area, may be found in the works of almost all the
naturalists who have visited it. I will give some of the principal
references in a note.[32]
[31] Ainsworth’s “Assyria and Babylon,” page 217.
[32] On Florida and the north shores of the Gulf of Mexico, Rogers’
“Report to Brit. Assoc.” volume iii., page 14.—On the shores of
Mexico, Humboldt, “Polit. Essay on New Spain,” volume i., page 62. (I
have also some corroborative facts with respect to the shores of
Mexico.)—Honduras and the Antilles, Lyell’s “Principles,” 5th edition,
volume iv., page 22.—Santa Cruz and Barbadoes, Prof. Hovey,
“Silliman’s Journal”, volume xxxv., page 74.—St. Domingo,
Courrojolles, “Journ de Phys.” tom. liv., page 106.—Bahamas, “United
Service Journal”, No. lxxi., pages 218 and 224. Jamaica, De la Beche,
“Geol. Man.” page 142.—Cuba, Taylor in “Lond. and Edin. Mag.” volume
xi., page 17. Dr. Daubeny also, at a meeting of the Geolog. Soc.,
orally described some very modern beds lying on the N.W. parts of
Cuba. I might have added many other less important references.
It is very remarkable on reviewing these details, to observe in how
many instances fringing-reefs round the shores, have coincided with the
existence on the land of upraised organic remains, which seem, from
evidence more or less satisfactory, to belong to a late tertiary
period. It may, however, be objected, that similar proofs of elevation,
perhaps, occur on the coasts coloured blue in our map: but this
certainly is not the case with the few following and doubtful
exceptions.
The entire area of the Red Sea appears to have been upraised within a
modern period; nevertheless I have been compelled (though on
unsatisfactory evidence, as given in the Appendix) to class the reefs
in the middle part, as barrier-reefs; should, however, the statements
prove accurate to the less height of the tertiary bed in this middle
part, compared with the northern and southern districts, we might well
suspect that it had subsided subsequently to the general elevation by
which the whole area has been upraised. Several authors[33] have stated
that they have observed shells and corals high up on the mountains of
the Society Islands,—a group encircled by barrier-reefs, and,
therefore, supposed to have subsided: at Tahiti Mr. Stutchbury found on
the apex of one of the highest mountains, between 5,000 and 7,000 feet
above the level of the sea, “a distinct and regular stratum of
semi-fossil coral.” At Tahiti, however, other naturalists, as well as
myself, have searched in vain at a low level near the coast, for
upraised shells or masses of coral-reef, where if present they could
hardly have been overlooked. From this fact, I concluded that probably
the organic remains strewed high up on the surface of the land, had
originally been embedded in the volcanic strata, and had subsequently
been washed out by the rain. I have since heard from the Rev. W. Ellis,
that the remains which he met with, were (as he believes)
interstratified with an argillaceous tuff; this likewise was the case
with the shells observed by the Rev. D. Tyerman at Huaheine. These
remains have not been specifically examined; they may, therefore, and
especially the stratum observed by Mr. Stutchbury at an immense height,
be contemporaneous with the first formation of the Society Islands, and
be of any degree of antiquity; or they may have been deposited at some
subsequent, but probably not very recent, period of elevation; for if
the period had been recent, the entire surface of the coast land of
these islands, where the reefs are so extensive, would have been coated
with upraised coral, which certainly is not the case. Two of the
Harvey, or Cook Islands, namely, Aitutaki and Manouai, are encircled by
reefs, which extend so far from the land, that I have coloured them
blue, although with much hesitation, as the space within the reef is
shallow, and the outline of the land is not abrupt. These two islands
consist of coral-rock; but I have no evidence of their recent
elevation, besides, the improbability of Mangaia, a fringed island in
the same group (but distant 170 miles), having retained its nearly
perfect atoll-like structure, during any immense lapse of time after
its upheaval. The Red Sea, therefore, is the only area in which we have
clear proofs of the recent elevation of a district, which, by our
theory (although the barrier-reefs are there not well characterised),
has lately subsided. But we have no reason to be surprised at
oscillation, of level of this kind having occasionally taken place.
There can be scarcely any doubt that Savage, Aurora,[34] and Mangaia
Islands, and several of the islands in the Friendly group, existed
originally as atolls, and these have undoubtedly since been upraised to
some height above the level of the sea; so that by our theory, there
has here, also, been an oscillation of level, —elevation having
succeeded subsidence, instead of, as in the middle part of the Red Sea
and at the Harvey Islands, subsidence having probably succeeded recent
elevation.
[33] Ellis, in his “Polynesian Researches,” was the first to call
attention to these remains (volume i., page 38), and the tradition of
the natives concerning them. See also Williams, “Nar. of Missionary
Enterprise,” page 21; also Tyerman and G. Bennett, “Journal of
Voyage,” volume i., page 213; also Mr. Couthouy’s “Remarks,” page 51;
but this principal fact, namely, that there is a mass of upraised
coral on the narrow peninsula of Tiarubu, is from hearsay evidence;
also Mr. Stutchbury, “West of England Journal,” No. i., page 54. There
is a passage in Von Zach, “Corres. Astronom.” volume x., page 266,
inferring an uprising at Tahiti, from a footpath now used, which was
formerly impassable; but I particularly inquired from several native
chiefs, whether they knew of any change of this kind, and they were
unanimous in giving me an answer in the negative.
[34] Aurora Island is described by Mr. Couthouy (“Remarks,” page 58);
it lies 120 miles north-east of Tahiti; it is not coloured in the
appended map, because it does not appear to be fringed by living
reefs. Mr. Couthouy describes its summit as “presenting a broad
table-land which declines a few feet towards the centre, where we may
suppose the lagoon to have been placed.” It is about two hundred feet
in height, and consists of reef-rock and conglomerate, with existing
species of coral embedded in it. The island has been elevated at two
successive periods; the cliffs being marked halfway up with a
horizontal water-worn line of deep excavations. Aurora Island seems
closely to resemble in structure Elizabeth Island, at the southern end
of the Low Archipelago.
It is an interesting fact, that Fais, which, from its composition,
form, height, and situation at the western end of the Caroline
Archipelago, one is strongly induced to believe existed before its
upheaval as an atoll, lies exactly in the prolongation of the curved
line of the Mariana group, which we know to be a line of recent
elevation. I may add, that Elizabeth Island, in the southern part of
the Low Archipelago, which seems to have had the same kind of origin as
Fais, lies near Pitcairn Island, the only one in this part of the ocean
which is high, and at the same time not surrounded by an encircling
barrier-reef.
_On the absence of active Volcanoes in the areas of subsidence, and on
their frequent presence in the areas of elevation._—Before making some
concluding remarks on the relations of the spaces coloured blue and
red, it will be convenient to consider the position on our map of the
volcanoes historically known to have been in action. It is impossible
not to be struck, first with the absence of volcanoes in the great
areas of subsidence tinted pale and dark blue,—namely, in the central
parts of the Indian Ocean, in the China Sea, in the sea between the
barriers of Australia and New Caledonia, in the Caroline, Marshall,
Gilbert, and Low Archipelagoes; and, secondly, with the coincidence of
the principal volcanic chains with the parts coloured red, which
indicates the presence of fringing-reefs; and, as we have just seen,
the presence in most cases of upraised organic remains of a modern
date. I may here remark that the reefs were all coloured before the
volcanoes were added to the map, or indeed before I knew of the
existence of several of them.
The volcano in Torres Strait, at the northern point of Australia, is
that which lies nearest to a large subsiding area, although situated
125 miles within the outer margin of the actual barrier-reef. The Great
Comoro Island, which probably contains a volcano, is only twenty miles
distant from the barrier-reef of Mohila; Ambil volcano, in the
Philippines, is distant only a little more than sixty miles from the
atoll-formed Appoo reef: and there are two other volcanoes in the map
within ninety miles of circles coloured blue. These few cases, which
thus offer partial exceptions to the rule, of volcanoes being placed
remote from the areas of subsidence, lie either near single and
isolated atolls, or near small groups of encircled islands; and these
by our theory can have, in few instances, subsided to the same amount
in depth or area, as groups of atolls. There is not one active volcano
within several hundred miles of an archipelago, or even a small group
of atolls. It is, therefore, a striking fact that in the Friendly
Archipelago, which owes its origin to the elevation of a group of
atolls, two volcanoes, and, perhaps, others[35] are known to be in
action: on the other hand, on several of the encircled islands in the
Pacific, supposed by our theory to have subsided, there are old craters
and streams of lava, which show the effects of past and ancient
eruptions. In these cases, it would appear as if the volcanoes had come
into action, and had become extinguished on the same spots, according
as the elevating or subsiding movements prevailed.
[35] See the note at p. 121, on the authorities for colouring the
volcanoes in the appended map.
There are some other coasts on the map, where volcanoes in a state of
action concur with proofs of recent elevation, besides those coloured
red from being fringed by coral-reefs. Thus I hope to show in a future
volume, that nearly the whole line of the west coast of South America,
which forms the greatest volcanic chain in the world, from near the
equator for a space of between 2,000 and 3,000 miles southward, has
undergone an upward movement during a late geological period. The
islands on the north-western shores of the Pacific, which form the
second greatest volcanic chain, are very imperfectly known; but Luzon,
in the Philippines, and the Loo Choo Islands, have been recently
elevated; and at Kamtschatka[36] there are extensive tertiary beds of
modern date. Evidence of the same nature, but not very satisfactory,
may be detected in Northern New Zealand where there are two volcanoes.
The co-existence in other parts of the world of active volcanoes, with
upraised beds of a modern tertiary origin, will occur to every
geologist.[37] Nevertheless, until it could be shown that volcanoes
were inactive, or did not exist in subsiding areas, the conclusion that
their distribution depended on the nature of the subterranean movements
in progress, would have been hazardous. But now, viewing the appended
map, it may, I think, be considered as almost established, that
volcanoes are often (not necessarily always) present in those areas
where the subterranean motive power has lately forced, or is now
forcing outwards, the crust of the earth, but that they are invariably
absent in those, where the surface has lately subsided or is still
subsiding.[38]
[36] At Sedanka, in latitude 58° N. (Von Buch’s “Descrip. des Isles
Canaries,” page 455). In a forthcoming part, I shall give the evidence
referred to with respect to the elevation of New Zealand.
[37] During the subterranean disturbances which took place in Chile,
in 1835, I have shown (“Geolog. Trans.” 2nd Ser., vol. v., page 606)
that at the same moment that a large district was upraised, volcanic
matter burst forth at widely separated points, through both new and
old vents.
[38] We may infer from this rule, that in any old deposit, which
contains interstratified beds of erupted matter, there was at the
period, and in the area of its formation, a _tendency_ to an upward
movement in the earth’s surface, and certainly no movement of
subsidence.
_On the relations of the areas of Subsidence and Elevation_.—The
immense surfaces on the map, which, both by our theory and by the plain
evidence of upraised marine remains, have undergone a change of level
either downwards or upwards during a late period, is a most remarkable
fact. The existence of continents shows that the areas have been
immense which at some period have been upraised; in South America we
may feel sure, and on the north-western shores of the Indian Ocean we
may suspect, that this rising is either now actually in progress, or
has taken place quite recently. By our theory, we may conclude that the
areas are likewise immense which have lately subsided, or, judging from
the earthquakes occasionally felt and from other appearances, are now
subsiding. The smallness of the scale of our map should not be
overlooked: each of the squares on it contains (not allowing for the
curvature of the earth) 810,000 square miles. Look at the space of
ocean from near the southern end of the Low Archipelago to the northern
end of the Marshall Archipelago, a length of 4,500 miles, in which, as
far as is known, every island, except Aurora which lies just without
the Low Archipelago, is atoll-formed. The eastern and western
boundaries of our map are continents, and they are rising areas: the
central spaces of the great Indian and Pacific Oceans, are mostly
subsiding; between them, north of Australia, lies the most broken land
on the globe, and there the rising parts are surrounded and penetrated
by areas of subsidence,[39] so that the prevailing movements now in
progress, seem to accord with the actual states of surface of the great
divisions of the world.
[39] I suspect that the Arru and Timor-laut Islands present an
included small area of subsidence, like that of the China Sea, but I
have not ventured to colour them from my imperfect information, as
given in the Appendix.
The blue spaces on the map are nearly all elongated; but it does not
necessarily follow from this (a caution, for which I am indebted to Mr.
Lyell), that the areas of subsidence were likewise elongated; for the
subsidence of a long, narrow space of the bed of the ocean, including
in it a transverse chain of mountains, surmounted by atolls, would only
be marked on the map by a transverse blue band. But where a chain of
atolls and barrier-reefs lies in an elongated area, between spaces
coloured red, which therefore have remained stationary or have been
upraised, this must have resulted either from the area of subsidence
having originally been elongated (owing to some tendency in the earth’s
crust thus to subside), or from the subsiding area having originally
been of an irregular figure, or as broad as long, and having since been
narrowed by the elevation of neighbouring districts. Thus the areas,
which subsided during the formation of the great north and south lines
of atolls in the Indian Ocean,—of the east and west line of the
Caroline atolls,—and of the north-west and south-east line of the
barrier-reefs of New Caledonia and Louisiade, must have originally been
elongated, or if not so, they must have since been made elongated by
elevations, which we know to belong to a recent period.
I infer from Mr. Hopkins’ researches,[40] that for the formation of a
long chain of mountains, with few lateral spurs, an area elongated in
the same direction with the chain, must have been subjected to an
elevatory movement. Mountain-chains, however, when already formed,
although running in very different directions, it seems,[41] may be
raised together by a widely-acting force: so, perhaps, mountain-chains
may subside together. Hence, we cannot tell, whether the Caroline and
Marshall Archipelagoes, two groups of atolls running in different
directions and meeting each other, have been formed by the subsidence
of two areas, or of one large area, including two distinct lines of
mountains. We have, however, in the southern prolongation of the
Mariana Islands, probable evidence of a line of recent elevation having
intersected one of recent subsidence. A view of the map will show that,
generally, there is a tendency to alternation in the parallel areas
undergoing opposite kinds of movement; as if the sinking of one area
balanced the rising of another.
[40] “Researches in Physical Geology,” Transact. Cambridge Phil. Soc.,
volume vi, part i.
[41] For instance in S. America from latitude 34°, for very many
degrees southward there are upraised beds containing recent species of
shells, on both the Atlantic and Pacific side of the continent, and
from the gradual ascent of the land, although with very unequal
slopes, on both sides towards the Cordillera, I think it can hardly be
doubted that the entire width has been upraised in mass within the
recent period. In this case the two W.N.W. and E.S.E. mountain-lines,
namely the Sierra Ventana and the S. Tapalguen, and the great north
and south line of the Cordillera have been together raised. In the
West Indies the N. and S. line of the Eastern Antilles, and the E. and
W. line of Jamaica, appear both to have been upraised within the
latest geological period.
The existence in many parts of the world of high table-land, proves
that large surfaces have been upraised in mass to considerable heights
above the level of the ocean; although the highest points in almost
every country consist of upturned strata, or erupted matter: and from
the immense spaces scattered with atolls, which indicate that land
originally existed there, although not one pinnacle now remains above
the level of the sea, we may conclude that wide areas have subsided to
an amount, sufficient to bury not only any formerly existing
table-land, but even the heights formed by fractured strata, and
erupted matter. The effects produced on the land by the later elevatory
movements, namely, successively rising cliffs, lines of erosion, and
beds of literal shells and pebbles, all requiring time for their
production, prove that these movements have been very slow; we can,
however, infer this with safety, only with respect to the few last
hundred feet of rise. But with reference to the whole vast amount of
subsidence, necessary to have produced the many atolls widely scattered
over immense spaces, it has already been shown (and it is, perhaps, the
most interesting conclusion in this volume), that the movements must
either have been uniform and exceedingly slow, or have been effected by
small steps, separated from each other by long intervals of time,
during which the reef-constructing polypifers were able to bring up
their solid frameworks to the surface. We have little means of judging
whether many considerable oscillations of level have generally occurred
during the elevation of large tracts; but we know, from clear
geological evidence, that this has frequently taken place; and we have
seen on our map, that some of the same islands have both subsided and
been upraised. I conclude, however, that most of the large blue spaces,
have subsided without many and great elevatory oscillations, because
only a few upraised atolls have been observed: the supposition that
such elevations have taken place, but that the upraised parts have been
worn down by the surf, and thus have escaped observation, is overruled
by the very considerable depth of the lagoons of all the larger atolls;
for this could not have been the case, if they had suffered repeated
elevations and abrasion. From the comparative observations made in
these latter pages, we may finally conclude, that the subterranean
changes which have caused some large areas to rise, and others to
subside, have acted in a very similar manner.
_Recapitulation_.—In the three first chapters, the principal kinds of
coral-reefs were described in detail, and they were found to differ
little, as far as relates to the actual surface of the reef. An atoll
differs from an encircling barrier-reef only in the absence of land
within its central expanse; and a barrier-reef differs from a
fringing-reef, in being placed at a much greater distance from the land
with reference to the probable inclination of its submarine foundation,
and in the presence of a deep-water lagoon-like space or moat within
the reef. In the fourth chapter the growing powers of the
reef-constructing polypifers were discussed; and it was shown, that
they cannot flourish beneath a very limited depth. In accordance with
this limit, there is no difficulty respecting the foundations on which
fringing-reefs are based; whereas, with barrier-reefs and atolls, there
is a great apparent difficulty on this head; in barrier-reefs from the
improbability of the rock of the coast or of banks of sediment
extending, in every instance, so far seaward within the required
depth;—and in atolls, from the immensity of the spaces over which they
are interspersed, and the apparent necessity for believing that they
are all supported on mountain-summits, which although rising very near
to the surface-level of the sea, in no one instance emerge above it. To
escape this latter most improbable admission, which implies the
existence of submarine chains of mountains of almost the same height,
extending over areas of many thousand square miles, there is but one
alternative; namely, the prolonged subsidence of the foundations, on
which the atolls were primarily based, together with the upward growth
of the reef-constructing corals. On this view every difficulty
vanishes; fringing reefs are thus converted into barrier-reefs; and
barrier-reefs, when encircling islands, are thus converted into atolls,
the instant the last pinnacle of land sinks beneath the surface of the
ocean.
Thus the ordinary forms and certain peculiarities in the structure of
atolls and barrier-reefs can be explained;—namely, the wall-like
structure on their inner sides, the basin or ring-like shape both of
the marginal and central reefs in the Maldiva atolls—the union of some
atolls as if by a ribbon—the apparent disseverment of others—and the
occurrence, in atolls as well as in barrier-reefs, of portions of reef,
and of the whole of some reefs, in a dead and submerged state, but
retaining the outline of living reefs. Thus can be explained the
existence of breaches through barrier-reefs in front of valleys, though
separated from them by a wide space of deep water; thus, also, the
ordinary outline of groups of atolls and the relative forms of the
separate atolls one to another; thus can be explained the proximity of
the two kinds of reefs formed during subsidence, and their separation
from the spaces where fringing-reefs abound. On searching for other
evidence of the movements supposed by our theory, we find marks of
change in atolls and in barrier-reefs, and of subterranean disturbances
under them; but from the nature of things, it is scarcely possible to
detect any direct proofs of subsidence, although some appearances are
strongly in favour of it. On the fringed coasts, however, the presence
of upraised marine bodies of a recent epoch, plainly show, that these
coasts, instead of having remained stationary, which is all that can be
directly inferred from our theory, have generally been elevated.
Finally, when the two great types of structure, namely barrier-reefs
and atolls on the one hand, and fringing-reefs on the other, were laid
down in colours on our map, a magnificent and harmonious picture of the
movements, which the crust of the earth has within a late period
undergone, is presented to us. We there see vast areas rising, with
volcanic matter every now and then bursting forth through the vents or
fissures with which they are traversed. We see other wide spaces slowly
sinking without any volcanic outburst, and we may feel sure, that this
sinking must have been immense in amount as well as in area, thus to
have buried over the broad face of the ocean every one of those
mountains, above which atolls now stand like monuments, marking the
place of their former existence. Reflecting how powerful an agent with
respect to denudation, and consequently to the nature and thickness of
the deposits in accumulation, the sea must ever be, when acting for
prolonged periods on the land, during either its slow emergence or
subsidence; reflecting, also, on the final effects of these movements
in the interchange of land and ocean-water on the climate of the earth,
and on the distribution of organic beings, I may be permitted to hope,
that the conclusions derived from the study of coral-formations,
originally attempted merely to explain their peculiar forms, may be
thought worthy of the attention of geologists.
APPENDIX.
CONTAINING A DETAILED DESCRIPTION OF THE REEFS AND ISLANDS IN PLATE
III.
In the beginning of the last chapter I stated the principles on which
the map is coloured. There only remains to be said, that it is an exact
copy of one by M. C. Gressier, published by the Dépôt général de la
Marine, in 1835. The names have been altered into English, and the
longitude has been reduced to that of Greenwich. The colours were first
laid down on accurate charts, on a large scale. The data, on which the
volcanoes historically known to have been in action, have been marked
with vermillion, were given in a note to the last chapter. I will
commence my description on the eastern side of the map, and will
describe each group of islands consecutively, proceeding westward
across the Pacific and Indian Oceans, but ending with the West Indies.
The WESTERN SHORES OF AMERICA appear to be entirely without
coral-reefs; south of the equator the survey of the “Beagle”, and north
of it, the published charts show that this is the case. Even in the Bay
of _Panama_, where corals flourish, there are no true coral-reefs, as I
have been informed by Mr. Lloyd. There are no coral-reefs in the
_Galapagos_ Archipelago, as I know from personal inspection; and I
believe there are none on the _Cocos, Revilla-gigedo_, and other
neighbouring islands. _Clipperton_ rock, 10° N., 109° W., has lately
been surveyed by Captain Belcher; in form it is like the crater of a
volcano. From a drawing appended to the MS. plan in the Admiralty, it
evidently is not an atoll. The eastern parts of the Pacific present an
enormous area, without any islands, except _Easter_, and _Sala_, and
_Gomez_ Islands, which do not appear to be surrounded by reefs.
The LOW ARCHIPELAGO.—This group consists of about eighty atolls: it
will be quite superfluous to refer to descriptions of each. In
D’Urville and Lottin’s chart, one island (_Wolchonsky_) is written with
a capital letter, signifying, as explained in a former chapter, that it
is a high island; but this must be a mistake, as the original chart by
Bellinghausen shows that it is a true atoll. Captain Beechey says of
the thirty-two groups which he examined (of the greater number of which
I have seen beautiful MS. charts in the Admiralty), that twenty-nine
now contain lagoons, and he believes the other three originally did.
Bellinghausen (see an account of his Russian voyage, in the “Biblioth.
des Voyages,” 1834, page 443) says, that the seventeen islands which he
discovered resembled each other in structure, and he has given charts
on a large scale of all of them. Kotzebue has given plans of several;
Cook and Bligh mention others; a few were seen during the voyage of the
“Beagle”; and notices of other atolls are scattered through several
publications. The _Actæon_ group in this archipelago has lately been
discovered (“Geographical Journal”, volume vii., page 454); it consists
of three small and low islets, one of which has a lagoon. Another
lagoon-island has been discovered (“Naut. Mag.” 1839, page 770), in 22°
4′ S., and 136° 20′ W. Towards the S.E. part of the group, there are
some islands of different formation: _Elizabeth_ Island is described by
Beechey (page 46, 4to edition) as fringed by reefs, at the distance of
between two and three hundred yards; coloured red. _Pitcairn_ Island,
in the immediate neighbourhood, according to the same authority, has no
reefs of any kind, although numerous pieces of coral are thrown up on
the beach; the sea close to its shore is very deep (see “Zool. of
Beechey’s Voyage,” page 164); it is left uncoloured. _Gambier_ Islands
(see Plate I., Figure 8), are encircled by a barrier-reef; the greatest
depth within is thirty-eight fathoms; coloured pale blue. _Aurora_
Island, which lies N.E. of Tahiti close to the large space coloured
dark blue in the map, has been already described in a note (page 71),
on the authority of Mr. Couthouy; it is an upraised atoll, but as it
does not appear to be fringed by living reefs, it is left uncoloured.
The SOCIETY Archipelago is separated by a narrow space from the Low
Archipelago; and in their parallel direction they manifest some
relation to each other. I have already described the general character
of the reefs of these fine encircled islands. In the “Atlas of the
‘Coquille’s’ Voyage” there is a good general chart of the group, and
separate plans of some of the islands. _Tahiti_, the largest island in
the group, is almost surrounded, as seen in Cook’s chart, by a reef
from half a mile to a mile and a half from the shore, with from ten to
thirty fathoms within it. Some considerable submerged reefs lying
parallel to the shore, with a broad and deep space within, have lately
been discovered (“Naut. Mag.” 1836, page 264) on the N.E. coast of the
island, where none are laid down by Cook. At _Eimeo_ the reef “which
like a ring surrounds it, is in some places one or two miles distant
from the shore, in others united to the beach” (Ellis, “Polynesian
Researches,” volume i., page 18, 12mo edition). Cook found deep water
(twenty fathoms) in some of the harbours within the reef. Mr. Couthouy,
however, states (“Remarks,” page 45) that both at Tahiti and Eimeo, the
space between the barrier-reef and the shore, has been almost filled
up,—“a nearly continuous fringing-reef surrounding the island, and
varying from a few yards to rather more than a mile in width, the
lagoons merely forming canals between this and the sea-reef,” that is
the barrier-reef. _Tapamanoa_ is surrounded by a reef at a considerable
distance from the shore; from the island being small it is breached, as
I am informed by the Rev. W. Ellis, only by a narrow and crooked boat
channel. This is the lowest island in the group, its height probably
not exceeding 500 feet. A little way north of Tahiti, the low
coral-islets of _Teturoa_ are situated; from the description of them
given me by the Rev. J. Williams (the author of the “Narrative of
Missionary Enterprise”), I should have thought they had formed a small
atoll, and likewise from the description given by the Rev. D. Tyerman
and G. Bennett (“Journal of Voyage and Travels,” volume i., page 183),
who say that ten low coral-islets “are comprehended within one general
reef, and separated from each other by interjacent lagoons;” but as Mr.
Stutchbury (“West of England Journal,” volume i., page 54) describes it
as consisting of a mere narrow ridge, I have left it uncoloured.
_Maitea_, eastward of the group, is classed by Forster as a high
encircled island; but from the account given by the Rev. D. Tyerman and
G. Bennett (volume i., page 57) it appears to be an exceedingly abrupt
cone, rising from the sea without any reef; I have left it uncoloured.
It would be superfluous to describe the northern islands in this group,
as they may be well seen in the chart accompanying the 4to edition of
Cook’s “Voyages,” and in the “Atlas of the ‘Coquille’s’ Voyage.”
_Maurua_ is the only one of the northern islands, in which the water
within the reef is not deep, being only four and a half fathoms; but
the great width of the reef, stretching three miles and a half
southward of the land (which is represented in the drawing in the
“Atlas of the ‘Coquille’s’ Voyage” as descending abruptly to the water)
shows, on the principle explained in the beginning of the last chapter,
that it belongs to the barrier class. I may here mention, from
information communicated to me by the Rev. W. Ellis, that on the N.E.
side of _Huaheine_ there is a bank of sand, about a quarter of a mile
wide, extending parallel to the shore, and separated from it by an
extensive and deep lagoon; this bank of sand rests on coral-rock, and
undoubtedly was originally a living reef. North of Bolabola lies the
atoll of _Toubai_ (Motou-iti of the “‘Coquille’s’ Atlas”) which is
coloured dark blue; the other islands, surrounded by barrier-reefs, are
pale blue; three of them are represented in Figures 3, 4, and 5, in
Plate I. There are three low coral-groups lying a little E. of the
Society Archipelago, and almost forming part of it, namely
_Bellinghausen_, which is said by Kotzebue (“Second Voyage,” volume
ii., page 255), to be a lagoon-island; _Mopeha_, which, from Cook’s
description (“Second Voyage,” book iii., chapter i.), no doubt is an
atoll; and the _Scilly_ Islands, which are said by Wallis (“Voyage,”
chapter ix.) to form a _group_ of _low_ islets and shoals, and,
therefore, probably, they compose an atoll: the two former have been
coloured blue, but not the latter.
MENDANA or MARQUESAS Group.—These islands are entirely without reefs,
as may be seen in Krusenstern’s Atlas, making a remarkable contrast
with the adjacent group of the Society Islands. Mr. F.D. Bennett has
given some account of this group, in the seventh volume of the
“Geographical Journal”. He informs me that all the islands have the
same general character, and that the water is very deep close to their
shores. He visited three of them, namely, _Dominicana_, _Christiana_,
and _Roapoa;_ their beaches are strewed with rounded masses of coral,
and although no regular reefs exist, yet the shore is in many places
lined by coral-rock, so that a boat grounds on this formation. Hence
these islands ought probably to come within the class of fringed
islands and be coloured red; but as I am determined to err on the
cautious side, I have left them uncoloured.
COOK or HARVEY and AUSTRAL ISLAND.—_Palmerston_ Island is minutely
described as an atoll by Captain Cook during his voyage in 1774;
coloured blue. _Aitutaki_ was partially surveyed by the “Beagle” (see
map accompanying “Voyages of ‘Adventure’ and ‘Beagle’”); the land is
hilly, sloping gently to the beach; the highest point is 360 feet; on
the southern side the reef projects five miles from the land: off this
point the “Beagle” found no bottom with 270 fathoms: the reef is
surmounted by many low coral-islets. Although within the reef the water
is exceedingly shallow, not being more than a few feet deep, as I am
informed by the Rev. J. Williams, nevertheless, from the great
extension of this reef into a profoundly deep ocean, this island
probably belongs, on the principle lately adverted to, to the barrier
class, and I have coloured it pale blue; although with much
hesitation.—_Manouai_ or _Harvey_ Island. The highest point is about
fifty feet: the Rev. J. Williams informs me that the reef here,
although it lies far from the shore, is less distant than at Aitutaki,
but the water within the reef is rather deeper: I have also coloured
this pale blue with many doubts.—Round _Mitiaro_ Island, as I am
informed by Mr. Williams, the reef is attached to the shore; coloured
red. —_Mauki_ or Maouti; the reef round this island (under the name of
Parry Island, in the “Voyage of H.M.S. ‘Blonde’,” page 209) is
described as a coral-flat, only fifty yards wide, and two feet under
water. This statement has been corroborated by Mr. Williams, who calls
the reef attached; coloured red.—_Aitu_, or Wateeo; a moderately
elevated hilly island, like the others of this group. The reef is
described in Cook’s “Voyage,” as attached to the shore, and about one
hundred yards wide; coloured red.—_Fenoua-iti;_ Cook describes this
island as very low, not more than six or seven feet high (volume i.,
book ii., chapter iii, 1777); in the chart published in the
“‘Coquille’s’ Atlas,” a reef is engraved close to the shore: this
island is not mentioned in the list given by Mr. Williams (page 16) in
the “Narrative of Missionary Enterprise;” nature doubtful. As it is so
near Atiu, it has been unavoidably coloured red.— _Rarotonga;_ Mr.
Williams informs me that it is a lofty basaltic island with an attached
reef; coloured red.—There are three islands, _Rourouti_, _Roxburgh_,
and _Hull_, of which I have not been able to obtain any account, and
have left them uncoloured. Hull Island, in the French chart, is written
with small letters as being low.—_Mangaia;_ height about three hundred
feet; “the surrounding reef joins the shore” (Williams, “Narrative,”
page 18); coloured red.—_Rimetara;_ Mr. Williams informs me that the
reef is rather close to the shore; but, from information given me by
Mr. Ellis, the reef does not appear to be quite so closely attached to
it as in the foregoing cases: the island is about three hundred feet
high (“Naut. Mag.” 1839, page 738); coloured red.—_Rurutu;_ Mr.
Williams and Mr. Ellis inform me that this island has an attached reef;
coloured red. It is described by Cook under the name of Oheteroa: he
says it is not surrounded, like the neighbouring islands by a reef; he
must have meant a distant reef.—_Toubouai;_ in Cook’s chart (“Second
Voyage,” volume ii., page 2) the reef is laid down in part one mile,
and in part two miles from the shore. Mr. Ellis (“Polynes. Res.” volume
iii., page 381) says the low land round the base of the island is very
extensive; and this gentleman informs me that the water within the reef
appears deep; coloured blue.—_Raivaivai_, or Vivitao; Mr. Williams
informs me that the reef is here distant: Mr. Ellis, however, says that
this is certainly not the case on one side of the island; and he
believes that the water within the reef is not deep; hence I have left
it uncoloured.—_Lancaster_ Reef, described in “Naut. Mag.” 1833 (page
693), as an extensive crescent-formed coral-reef. I have not coloured
it.—_Rapa_, or Oparree; from the accounts given of it by Ellis and
Vancouver, there does not appear to be any reef.—_I. de Bass_ is an
adjoining island, of which I cannot find any account.—_Kemin_ Island;
Krusenstern seems hardly to know its position, and gives no further
particulars.
ISLANDS BETWEEN _the Low and Gilbert Archipelagoes_.
_Caroline_ Island (10° S., 150° W.) is described by Mr. F.D. Bennett
(“Geographical Journal”, volume vii., page 225) as containing a fine
lagoon; coloured blue.—_Flint_ Island (11° S., 151° W.); Krusenstern
believes that it is the same with _Peregrino_, which is described by
Quiros (Burney’s “Chron. Hist.” volume ii., page 283) as “a cluster of
small islands connected by a reef, and forming a lagoon in the middle;”
coloured blue.—_Wostock_ is an island a little more than half a mile in
diameter, and apparently quite flat and low, and was discovered by
Bellinghausen; it is situated a little west of Caroline Island, but it
is not placed on the French charts; I have not coloured it, although I
entertain little doubt from the chart of Bellinghausen, that it
originally contained a small lagoon.—_Penrhyn_ Island (9° S., 158° W.);
a plan of it in the “Atlas of the First Voyage” of Kotzebue, shows that
it is an atoll; blue.— _Starbuck_ Island (5° S., 156° W.) is described
in Byron’s “Voyage in the ‘Blonde’” (page 206) as formed of a flat
coral-rock, with no trees; the height not given; not coloured.—_Malden_
Island (4° S., 154° W.); in the same voyage (page 205) this island is
said to be of coral formation, and no part above forty feet high; I
have not ventured to colour it, although, from being of
coral-formation, it is probably fringed; in which case it should be
red.—_Jarvis_, or _Bunker_ Island (0° 20′ S., 160° W.) is described by
Mr. F.D. Bennett (“Geographical Journal”, volume vii., page 227) as a
narrow, low strip of coral-formation; not coloured.—_Brook_, is a small
low island between the two latter; the position, and perhaps even the
existence of it is doubtful; not coloured.—_Pescado_ and _Humphrey_
Islands; I can find out nothing about these islands, except that the
latter appears to be small and low; not coloured.—_Rearson_, or Grand
Duke Alexander’s (10 S., 161° W.); an atoll, of which a plan is given
by Bellinghausen; blue.— _Souvoroff_ Islands (13° S., 163° W.); Admiral
Krusenstern, in the most obliging manner, obtained for me an account of
these islands from Admiral Lazareff, who discovered them. They consist
of five very low islands of coral-formation, two of which are connected
by a reef, with deep water close to it. They do not surround a lagoon,
but are so placed that a line drawn through them includes an oval
space, part of which is shallow; these islets, therefore, probably once
(as is the case with some of the islands in the Caroline Archipelago)
formed a single atoll; but I have not coloured them.—_Danger_ Island
(10° S., 166° W.); described as low by Commodore Byron, and more lately
surveyed by Bellinghausen; it is a small atoll with three islets on it;
blue.—_Clarence_ Island (9° S., 172° W.); discovered in the “Pandora”
(G. Hamilton’s “Voyage,” page 75): it is said, “in running along the
land, we saw several canoes crossing the _lagoons;_” as this island is
in the close vicinity of other low islands, and as it is said, that the
natives make reservoirs of water in old cocoa-nut trees (which shows
the nature of the land), I have no doubt it is an atoll, and have
coloured it blue. _York_ Island (8° S., 172° W.) is described by
Commodore Byron (chapter x. of his “Voyage”) as an atoll;
blue.—_Sydney_ Island (4° S., 172° W.) is about three miles in
diameter, with its interior occupied by a lagoon (Captain Tromelin,
“Annal. Marit.” 1829, page 297); blue.—_Phœnix_ Island (4° S., 171° W.)
is nearly circular, low, sandy, not more than two miles in diameter,
and very steep outside (Tromelin, “Annal. Marit.” 1829, page 297); it
may be inferred that this island originally contained a lagoon, but I
have not coloured it.—_New Nantucket_ (0° 15′ N., 174° W.). From the
French chart it must be a low island; I can find nothing more about it
or about _Mary_ Island; both uncoloured.—_Gardner_ Island (5° S., 174°
W.) from its position is certainly the same as _Kemin_ Island described
(Krusenstern, page 435, Appen. to Mem., published 1827) as having a
lagoon in its centre; blue.
ISLANDS SOUTH _of the Sandwich Archipelago_.
_Christmas_ Island (2° N., 157° W.). Captain Cook, in his “Third
Voyage” (Volume ii., chapter x.), has given a detailed account of this
atoll. The breadth of the islets on the reef is unusually great, and
the sea near it does not deepen so suddenly as is generally the case.
It has more lately been visited by Mr. F.D. Bennett (“Geographical
Journal,” volume vii., page 226); and he assures me that it is low and
of coral-formation: I particularly mention this, because it is engraved
with a capital letter, signifying a high island, in D’Urville and
Lottin’s chart. Mr. Couthouy, also, has given some account of it
(“Remarks,” page 46) from the Hawaiian “Spectator”; he believes it has
lately undergone a small elevation, but his evidence does not appear to
me satisfactory; the deepest part of the lagoon is said to be only ten
feet; nevertheless, I have coloured it blue.—_Fanning_ Island (4° N.,
158° W.) according to Captain Tromelin (“Ann. Maritim.” 1829, page
283), is an atoll: his account as observed by Krusenstern, differs from
that given in Fanning’s “Voyage” (page 224), which, however, is far
from clear; coloured blue.— _Washington_ Island (4° N., 159° W.) is
engraved as a low island in D’Urville’s chart, but is described by
Fanning (page 226) as having a much greater elevation than Fanning
Island, and hence I presume it is not an atoll; not coloured.—_Palmyra_
Island (6° N., 162° W.) is an atoll divided into two parts
(Krusenstern’s “Mem. Suppl.” page 50, also Fanning’s “Voyage,” page
233); blue.—_Smyth’s_ or Johnston’s Islands (17° N., 170° W.). Captain
Smyth, R.N., has had the kindness to inform me that they consist of two
very low, small islands, with a dangerous reef off the east end of
them. Captain Smyth does not recollect whether these islets, together
with the reef, surrounded a lagoon; uncoloured.
SANDWICH ARCHIPELAGO.—_Hawaii;_ in the chart in Freycinet’s “Atlas,”
small portions of the coast are fringed by reefs; and in the
accompanying “Hydrog. Memoir,” reefs are mentioned in several places,
and the coral is said to injure the cables. On one side of the islet of
Kohaihai there is a bank of sand and coral with five feet water on it,
running parallel to the shore, and leaving a channel of about fifteen
feet deep within. I have coloured this island red, but it is very much
less perfectly fringed than others of the group.—_Maui;_ in Freycinet’s
chart of the anchorage of Raheina, two or three miles of coast are seen
to be fringed; and in the “Hydrog. Memoir,” “banks of coral along
shore” are spoken of. Mr. F.D. Bennett informs me that the reefs, on an
average, extend about a quarter of a mile from the beach; the land is
not very steep, and outside the reefs the sea does not become deep very
suddenly; coloured red.—_Morotoi_, I presume, is fringed: Freycinet
speaks of the breakers extending along the shore at a little distance
from it. From the chart, I believe it is fringed; coloured red.—_Oahu;_
Freycinet, in his “Hydrog. Memoir,” mentions some of the reefs. Mr.
F.D. Bennett informs me that the shore is skirted for forty or fifty
miles in length. There is even a harbour for ships formed by the reefs,
but it is at the mouth of a valley; red.—_Atooi_, in La Peyrouse’s
charts, is represented as fringed by a reef, in the same manner as Oahu
and Morotoi; and this, as I have been informed by Mr. Ellis, on part at
least of the shore, is of coral-formation: the reef does not leave a
deep channel within; red.—_Oneehow;_ Mr. Ellis believes that this
island is also fringed by a coral-reef: considering its close proximity
to the other islands, I have ventured to colour it red. I have in vain
consulted the works of Cook, Vancouver, La Peyrouse, and Lisiansky, for
any satisfactory account of the small islands and reefs, which lie
scattered in a N.W. line prolonged from the Sandwich group, and hence
have left them uncoloured, with one exception; for I am indebted to Mr.
F.D. Bennett for informing me of an atoll-formed reef, in latitude 28°
22′, longitude 178° 30′ W., on which the “Gledstanes” was wrecked in
1837. It is apparently of large size, and extends in a N.W. and S.E.
line: very few islets have been formed on it. The lagoon seems to be
shallow; at least, the deepest part which was surveyed was only three
fathoms. Mr. Couthouy (“Remarks,” page 38) describes this island under
the name of _Ocean_ island. Considerable doubts should be entertained
regarding the nature of a reef of this kind, with a very shallow
lagoon, and standing far from any other atoll, on account of the
possibility of a crater or flat bank of rock lying at the proper depth
beneath the surface of the water, thus affording a foundation for a
ring-formed coral-reef. I have, however, thought myself compelled, from
its large size and symmetrical outline, to colour it blue.
SAMOA or NAVIGATOR GROUP.—Kotzebue, in his “Second Voyage,” contrasts
the structure of these islands with many others in the Pacific, in not
being furnished with harbours for ships, formed by distant coral-reefs.
The Rev. J. Williams, however, informs me, that coral-reefs do occur in
irregular patches on the shores of these islands; but that they do not
form a continuous band, as round Mangaia, and other such perfect cases
of fringed islands. From the charts accompanying La Peyrouse’s
“Voyage,” it appears that the north shore of _Savaii_, _Maouna_,
_Orosenga_, and _Mannua_, are fringed by reefs. La Peyrouse, speaking
of Maouna (page 126), says that the coral-reef surrounding its shores,
almost touches the beach; and is breached in front of the little coves
and streams, forming passages for canoes, and probably even for boats.
Further on (page 159), he extends the same observation to all the
islands which he visited. Mr. Williams in his “Narrative,” speaks of a
reef going round a small island attached to _Oyolava_, and returning
again to it: all these islands have been coloured red.—A chart of
_Rose_ Island, at the extreme west end of the group, is given by
Freycinet, from which I should have thought that it had been an atoll;
but according to Mr. Couthouy (“Remarks,” page 43), it consists of a
reef, only a league in circuit, surmounted by a very few low islets;
the lagoon is very shallow, and is strewed with numerous large boulders
of volcanic rock. This island, therefore, probably consists of a bank
of rock, a few feet submerged, with the outer margin of its upper
surface fringed with reefs; hence it cannot be properly classed with
atolls, in which the foundations are always supposed to lie at a depth,
greater than that at which the reef-constructing polypifers can live;
not coloured.
_Beveridge Reef_, 20° S., 167° W., is described in the “Naut. Mag.”
(May 1833, page 442) as ten miles long in a N. and S. line, and eight
wide; “in the inside of the reef there appears deep water;” there is a
passage near the S.W. corner: this therefore seems to be a submerged
atoll, and is coloured blue.
_Savage_ Island, 19° S., 170° W., has been described by Cook and
Forster. The younger Forster (volume ii., page 163) says it is about
forty feet high: he suspects that it contains a low plain, which
formerly was the lagoon. The Rev. J. Williams informs me that the reef
fringing its shores, resembles that round Mangaia; coloured red.
FRIENDLY ARCHIPELAGO. _Pylstaart_ Island. Judging from the chart in
Freycinet’s “Atlas,” I should have supposed that it had been regularly
fringed; but as nothing is said in the “Hydrog. Memoir” (or in the
“Voyage” of Tasman, the discoverer) about coral-reefs, I have left it
uncoloured.—_Tongatabou:_ In the “Atlas of the Voyage of the
‘Astrolabe’,” the whole south side of the island is represented as
narrowly fringed by the same reef which forms an extensive platform on
the northern side. The origin of this latter reef, which might have
been mistaken for a barrier-reef, has already been attempted to be
explained, when giving the proofs of the recent elevation of this
island.— In Cook’s charts the little outlying island also of _Eoaigee_,
is represented as fringed; coloured red.—_Eoua_. I cannot make out from
Captain Cook’s charts and descriptions, that this island has any reef,
although the bottom of the neighbouring sea seems to be corally, and
the island itself is formed of coral-rock. Forster, however, distinctly
(“Observations,” page 14) classes it with high islands having reefs,
but it certainly is not encircled by a barrier-reef and the younger
Forster (“Voyage,” volume i., page 426) says, that “a bed of
coral-rocks surrounded the coast towards the landing-place.” I have
therefore classed it with the fringed islands and coloured it red. The
several islands lying N.W. of Tongatabou, namely _Anamouka_, _Komango_,
_Kotou_, _Lefouga_, _Foa_, &c., are seen in Captain Cook’s chart to be
fringed by reefs, in several of them are connected together. From the
various statements in the first volume of Cook’s “Third Voyage,” and
especially in the fourth and sixth chapters, it appears that these
reefs are of coral-formation, and certainly do not belong to the
barrier class; coloured red.—_Toufoua_ & _Kao_, forming the western
part of the group, according to Forster have no reefs; the former is an
active volcano.—_Vavao_. There is a chart of this singularly formed
island, by Espinoza: according to Mr. Williams it consists of
coral-rock: the Chevalier Dillon informs me that it is not fringed; not
coloured. Nor are the islands of _Latte_ and _Amargura_, for I have not
seen plans on a large scale of them, and do not know whether they are
fringed.
_Niouha_, 16° S., 174° W., or _Keppel_ Island of Wallis, or _Cocos_
Island. From a view and chart of this island given in Wallis’s “Voyage”
(4to edition) it is evidently encircled by a reef; coloured blue: it is
however remarkable that _Boscawen_ Island, immediately adjoining, has
no reef of any kind; uncoloured.
_Wallis Island_, 13° S., 176° W., a chart and view of this island in
Wallis’s “Voyage” (4to edition) shows that it is encircled. A view of
it in the “Naut. Mag.” July 1833, page 376, shows the same fact; blue.
_Alloufatou_, or _Horn_ Island, _Onouafu_, or _Proby_ Island, and
_Hunter_ Islands, lie between the Navigator and Fidji groups. I can
find no distinct accounts of them.
FIDJI or VITI GROUP.—The best chart of the numerous islands of this
group, will be found in the “Atlas of the ‘Astrolabe’s’ Voyage.” From
this, and from the description given in the “Hydrog. Memoir,”
accompanying it, it appears that many of these islands are bold and
mountainous, rising to the height of between 3,000 and 4,000 feet. Most
of the islands are surrounded by reefs, lying far from the land, and
outside of which the ocean appears very deep. The “Astrolabe” sounded
with ninety fathoms in several places about a mile from the reefs, and
found no bottom. Although the depth within the reef is not laid down,
it is evident from several expressions, that Captain D’Urville believes
that ships could anchor within, if passages existed through the outer
barriers. The Chevallier Dillon informs me that this is the case: hence
I have coloured this group blue. In the S.E. part lies _Batoa_, or
_Turtle_ Island of Cook (“Second Voyage,” volume ii., page 23, and
chart, 4to edition) surrounded by a coral-reef, “which in some places
extends two miles from the shore;” within the reef the water appears to
be deep, and outside it is unfathomable; coloured pale blue. At the
distance of a few miles, Captain Cook (Ibid., page 24) found a circular
coral-reef, four or five leagues in circuit, with deep water within;
“in short, the bank wants only a few little islets to make it exactly
like one of the half-drowned isles so often mentioned,”—namely, atolls.
South of Batoa, lies the high island of _Ono_, which appears in
Bellinghausen’s “Atlas” to be encircled; as do some other small islands
to the south; coloured pale blue; near Ono, there is an annular reef,
quite similar to the one just described in the words of Captain Cook;
coloured dark blue.
_Rotoumah_, 13° S., 179° E.—From the chart in Duperrey’s “Atlas,” I
thought this island was encircled, and had coloured it blue, but the
Chevallier Dillon assures me that the reef is only a shore or fringing
one; red.
_Independence_ Island, 10° S., 179° E., is described by Mr. G. Bennett,
(“United Service Journal,” 1831, part ii., page 197) as a low island of
coral-formation, it is small, and does not appear to contain a lagoon,
although an opening through the reef is referred to. A lagoon probably
once existed, and has since been filled up; left uncoloured.
ELLICE GROUP.—_Oscar_, _Peyster_, and _Ellice_ Islands are figured in
Arrowsmith’s “Chart of the Pacific” (corrected to 1832) as atolls, and
are said to be very low; blue.—_Nederlandisch_ Island. I am greatly
indebted to the kindness of Admiral Krusenstern, for sending me the
original documents concerning this island. From the plans given by
Captains Eeg and Khremtshenko, and from the detailed account given by
the former, it appears that it is a narrow coral-island, about two
miles long, containing a small lagoon. The sea is very deep close to
the shore, which is fronted by sharp coral-rocks. Captain Eeg compares
the lagoon with that of other coral-islands; and he distinctly says,
the land is “very low.” I have therefore coloured it blue. Admiral
Krusenstern (“Memoir on the Pacific,” Append., 1835) states that its
shores are eighty feet high; this probably arose from the height of the
cocoa-nut trees, with which it is covered, being mistaken for land.
—_Gran Cocal_ is said in Krusenstern’s “Memoir,” to be low, and to be
surrounded by a reef; it is small, and therefore probably once
contained a lagoon; uncoloured.—_St. Augustin_. From a chart and view
of it, given in the “Atlas of the ‘Coquille’s’ Voyage,” it appears to
be a small atoll, with its lagoon partly filled up; coloured blue.
GILBERT GROUP.—The chart of this group, given in the “Atlas of the
‘Coquille’s’ Voyage,” at once shows that it is composed of ten well
characterised atolls. In D’Urville and Lottin’s chart, _Sydenham_ is
written with a capital letter, signifying that it is high; but this
certainly is not the case, for it is a perfectly characterised atoll,
and a sketch, showing how low it is, is given in the “‘Coquille’s’
Atlas.” Some narrow strip-like reefs project from the southern side of
_Drummond_ atoll, and render it irregular. The southern island of the
group is called _Chase_ (in some charts, _Rotches_); of this I can find
no account, but Mr. F.D. Bennett discovered (“Geographical Journal”,
volume vii., page 229), a low extensive island in nearly the same
latitude, about three degrees westward of the longitude assigned to
Rotches, but very probably it is the same island. Mr. Bennett informs
me that the man at the masthead reported an appearance of lagoon-water
in the centre; and, therefore, considering its position, I have
coloured it blue. —_Pitt_ Island, at the extreme northern point of the
group, is left uncoloured, as its exact position and nature is not
known.—_Byron_ Island, which lies a little to the eastward, does not
appear to have been visited since Commodore Byron’s voyage, and it was
then seen only from a distance of eighteen miles; it is said to be low;
uncoloured.
_Ocean_, _Pleasant_, and _Atlantic_ Islands all lie considerably to the
west of the Gilbert group: I have been unable to find any distinct
account of them. Ocean Island is written with small letters in the
French chart, but in Krusenstern’s “Memoir” it is said to be high.
MARSHALL GROUP.—We are well acquainted with this group from the
excellent charts of the separate islands, made during the two voyages
of Kotzebue: a reduced one of the whole group may be easily seen in
Krusenstern’s “Atlas,” and in Kotzebue’s “Second Voyage.” The group
consists (with the exception of two _little_ islands which probably
have had their lagoon filled up) of a double row of twenty-three large
and well-characterised atolls, from the examination of which Chamisso
has given us his well-known account of coral-formations. I include
_Gaspar-Rico_, or _Cornwallis_ Island in this group, which is described
by Chamisso (Kotzebue’s “First Voyage,” volume iii., page 179) “as a
low sickle-formed group, with mould only on the windward side.” Gaspard
Island is considered by some geographers as a distinct island lying
N.E. of the group, but it is not entered in the chart by Krusenstern;
left uncoloured. In the S.W. part of this group lies _Baring_ Island,
of which little is known (see Krusenstern’s “Appendix,” 1835, page
149). I have left it uncoloured; but _Boston_ Island I have coloured
blue, as it is described (Ibid.) as consisting of fourteen small
islands, which, no doubt, enclose a lagoon, as represented in a chart
in the “‘Coquille’s’ Atlas.”—Two islands, _Aur Kawen_ and _Gaspar
Rico_, are written in the French chart with capital letters; but this
is an error, for from the account given by Chamisso in Kotzebue’s
“First Voyage,” they are certainly low. The nature, position, and even
existence, of the shoals and small islands north of the Marshall group,
are doubtful.
NEW HEBRIDES.—Any chart, on even a small scale, of these islands, will
show that their shores are almost without reefs, presenting a
remarkable contrast with those of New Caledonia on the one hand, and
the Fidji group on the other. Nevertheless, I have been assured by Mr.
G. Bennett, that coral grows vigorously on their shores; as indeed,
will be further shown in some of the following notices. As, therefore,
these islands are not encircled, and as coral grows vigorously on their
shores, we might almost conclude, without further evidence, that they
were fringed, and hence I have applied the red colour with rather
greater freedom than in other instances.—_Matthew’s Rock_, an active
volcano, some way south of the group (of which a plan is given in the
“Atlas of the ‘Astrolabe’s’ Voyage”) does not appear to have reefs of
any kind about it.—_Annatom_, the southernmost of the Hebrides; from a
rough woodcut given in the “United Service Journal” (1831, part iii.,
page 190), accompanying a paper by Mr. Bennett, it appears that the
shore is fringed; coloured red.—_Tanna_. Forster, in his “Observations”
(page 22), says Tanna has on its shores coral-rock and madrepores; and
the younger Forster, in his account (volume ii., page 269) speaking of
the harbour says, the whole S.E. side consists of coral-reefs, which
are overflowed at high-water; part of the southern shore in Cook’s
chart is represented as fringed; coloured red.—_Immer_ is described
(“United Service Journal,” 1831, part iii., page 192) by Mr. Bennett as
being of moderate elevation, with cliffs appearing like sandstone:
coral grows in patches on its shore, but I have not coloured it; and I
mention these facts, because Immer might have been thought from
Forster’s classification (“Observations,” page 14), to have been a low
island or even an atoll.— _Erromango_ Island; Cook (“Second Voyage,”
volume ii., page 45, 4to edition) speaks of rocks everywhere _lining_
the coast, and the natives offered to haul his boat over the breakers
to the sandy beach: Mr. Bennett, in a letter to the Editor of the
“Singapore Chron.,” alludes to the _reefs_ on its shores. It may, I
think, be safely inferred from these passages that the shore is fringed
in parts by coral-reefs; coloured red.—_Sandwich_ Island. The east
coast is said (Cook’s “Second Voyage,” volume ii., page 41) to be low,
and to be guarded by a chain of breakers. In the accompanying chart it
is seen to be fringed by a reef; coloured red.—_Mallicollo_; Forster
speaks of the reef-bounded shore: the reef is about thirty yards wide,
and so shallow that a boat cannot pass over it. Forster also
(“Observations,” page 23) says, that the rocks of the sea-shore consist
of madrepore. In the plan of Sandwich harbour, the headlands are
represented as fringed; coloured red.—_Aurora_ and _Pentecost_ Islands,
according to Bougainville, apparently have no reefs; nor has the large
island of _S. Espiritu_, nor _Bligh_ Island or _Banks’_ Islands, which
latter lie to the N.E. of the Hebrides. But in none of these cases,
have I met with any detailed account of their shores, or seen plans on
a large scale; and it will be evident, that a fringing-reef of only
thirty or even a few hundred yards in width, is of so little importance
to navigation, that it will seldom be noticed, excepting by chance; and
hence I do not doubt that several of these islands, now left
uncoloured, ought to be red.
SANTA-CRUZ GROUP.—_Vanikoro_ (Figure 1, Plate I.) offers a striking
example of a barrier-reef: it was first described by the Chevalier
Dillon, in his voyage, and was surveyed in the “Astrolabe”; coloured
pale blue.—_Tikopia_ and _Fataka_ Islands appear, from the descriptions
of Dillon and D’Urville, to have no reefs; _Anouda_ is a low, flat
island, surrounded by cliffs (“‘Astrolabe’ Hydrog.” and Krusenstern,
“Mem.” volume ii., page 432); these are uncoloured. _Toupoua_
(_Otooboa_ of Dillon) is stated by Captain Tromelin (“Annales Marit.”
1829, page 289) to be almost entirely included in a reef, lying at the
distance of two miles from the shore. There is a space of three miles
without any reef, which, although indented with bays, offers no
anchorage from the extreme depth of the water close to the shore:
Captain Dillon also speaks of the reefs fronting this island; coloured
blue.— _Santa-Cruz_. I have carefully examined the works of Carteret,
D’Entrecasteaux, Wilson, and Tromelin, and I cannot discover any
mention of reefs on its shores; left uncoloured.—_Tinakoro_ is a
constantly active volcano without reefs.—_Mendana Isles_ (mentioned by
Dillon under the name of _Mammee_, etc.); said by Krusenstern to be
low, and intertwined with reefs. I do not believe they include a
lagoon; I have left them uncoloured.—_Duff’s_ Islands compose a small
group directed in a N.W. and S.E. band; they are described by Wilson
(page 296, “Miss. Voy.” 4to edition), as formed by bold-peaked land,
with the islands surrounded by coral-reefs, extending about half a mile
from the shore; at a distance of a mile from the reefs he found only
seven fathoms. As I have no reason for supposing there is deep water
within these reefs, I have coloured them red. _Kennedy_ Island, N.E. of
Duff’s. I have been unable to find any account of it.
NEW CALEDONIA.—The great barrier-reefs on the shores of this island
have already been described (Figure 5, Plate II.). They have been
visited by Labillardière, Cook, and the northern point by D’Urville;
this latter part so closely resembles an atoll that I have coloured it
dark blue. The _Loyalty_ group is situated eastward of this island;
from the chart and description given in the “Voyage of the
‘Astrolabe’,” they do not appear to have any reefs; north of this
group, there are some extensive low reefs (called _Astrolabe_ and
_Beaupré_,) which do not seem to be atoll-formed; these are left
uncoloured.
AUSTRALIAN BARRIER-REEF.—The limits of this great reef, which has
already been described, have been coloured from the charts of Flinders
and King. In the northern parts, an atoll-formed reef, lying outside
the barrier, has been described by Bligh, and is coloured dark blue. In
the space between Australia and New Caledonia, called by Flinders the
Corallian Sea, there are numerous reefs. Of these, some are represented
in Krusenstern’s “Atlas” as having an atoll-like structure; namely,
_Bampton_ shoal, _Frederic_, _Vine_ or Horse-shoe, and _Alert_ reefs;
these have been coloured dark blue.
LOUISIADE: the dangerous reefs which front and surround the western,
southern, and northern coasts of this so-called peninsula and
archipelago, seem evidently to belong to the barrier class. The land is
lofty, with a low fringe on the coast; the reefs are distant, and the
sea outside them profoundly deep. Nearly all that is known of this
group is derived from the labours of D’Entrecasteaux and Bougainville:
the latter has represented one continuous reef ninety miles long,
parallel to the shore, and in places as much as ten miles from it;
coloured pale blue. A little distance northward we have the _Laughlan_
Islands, the reefs round which are engraved in the “Atlas of the Voyage
of the ‘Astrolabe’,” in the same manner as in the encircled islands of
the Caroline Archipelago, the reef is, in parts, a mile and a half from
the shore, to which it does not appear to be attached; coloured blue.
At some little distance from the extremity of the Louisiade lies the
_Wells_ reef, described in G. Hamilton’s “Voyage in H.M.S. ‘Pandora’”
(page 100): it is said, “We found we had got embayed in a double reef,
which will soon be an island.” As this statement is only intelligible
on the supposition of the reef being crescent or horse-shoe formed,
like so many other submerged annular reefs, I have ventured to colour
it blue.
SALOMON ARCHIPELAGO: the chart in Krusenstern’s “Atlas” shows that
these islands are not encircled, and as coral appears from the works of
Surville, Bougainville, and Labillardière, to grow on their shores,
this circumstance, as in the case of the New Hebrides, is a presumption
that they are fringed. I cannot find out anything from
D’Entrecasteaux’s “Voyage,” regarding the southern islands of the
group, so have left them uncoloured.—_Malayta_ Island in a rough MS.
chart in the Admiralty has its northern shore fringed.—_Ysabel_ Island,
the N.E. part of this island, in the same chart, is also fringed:
Mendana, speaking (Burney, volume i., page 280) of an islet adjoining
the northern coast, says it is surrounded by reefs; the shores, also of
Port Praslin appear regularly fringed.—_Choiseul_ Island. In
Bougainville’s “Chart of Choiseul Bay,” parts of the shores are fringed
by coral-reefs.— _Bougainville_ Island. According to D’Entrecasteaux
the western shore abounds with coral-reefs, and the smaller islands are
said to be attached to the larger ones by reefs; all the
before-mentioned islands have been coloured red.—_Bouka_ Islands.
Captain Duperrey has kindly informed me in a letter that he passed
close round the northern side of this island (of which a plan is given
in his “Atlas of the ‘Coquille’s’ Voyage”), and that it was “garnie
d’une bande de récifs à fleur d’eau adherentes au rivage;” and he
infers, from the abundance of coral on the islands north and south of
Bouka, that the reef probably is of coral; coloured red.
Off the north coast of the Solomon Archipelago there are several small
groups which are little known; they appear to be low, and of
coral-formation; and some of them probably have an atoll-like
structure; the Chevallier Dillon, however, informs me that this is not
the case with the B. de _Candelaria_.—_Outong Java_, according to the
Spanish navigator, Maurelle, is thus characterised; but this is the
only one which I have ventured to colour blue.
NEW IRELAND.—The shores of the S.W. point of this island and some
adjoining islets, are fringed by reefs, as may be seen in the “Atlases
of the Voyages of the ‘Coquille’ and ‘Astrolabe’.” M. Lesson observes
that the reefs are open in front of each streamlet. The _Duke of
York’s_ Island is also fringed; but with regard to the other parts of
_New Ireland_, _New Hanover_, and the small islands lying northward, I
have been unable to obtain any information. I will only add that no
part of New Ireland appears to be fronted by distant reefs. I have
coloured red only the above specified portions.
NEW BRITAIN and the NORTHERN SHORE of NEW GUINEA.—From the charts in
the “Voyage of the ‘Astrolabe’,” and from the “Hydrog. Memoir,” it
appears that these coasts are entirely without reefs, as are the
_Schouten Islands_, lying close to the northern shore of New Guinea.
The western and south-western parts of New Guinea, will be treated of
when we come to the islands of the East Indian Archipelago.
ADMIRALTY GROUP.—From the accounts by Bougainville, Maurelle,
D’Entrecasteaux, and the scattered notices collected by Horsburgh, it
appears, that some of the many islands composing it, are high, with a
bold outline; and others are very low, small and interlaced with reefs.
All the high islands appear to be fronted by distant reefs rising
abruptly from the sea, and within some of which there is reason to
believe that the water is deep. I have therefore little doubt they are
of the barrier class.—In the southern part of the group we have
_Elizabeth Island_, which is surrounded by a reef at the distance of a
mile; and two miles eastward of it (Krusenstern, “Append.” 1835, page
42) there is a little island containing a lagoon.—Near here, also lies
_Circular-reef_ (Horsburgh, “Direct.” volume i., page 691, 4th
edition), “three or four miles in diameter having deep water inside
with an opening at the N.N.W. part, and on the outside steep to.” I
have from these data, coloured the group pale blue, and _circular-reef_
dark blue.—the _Anachorites_, _Echequier_, and _Hermites_, consist of
innumerable low islands of coral-formation, which probably have
atoll-like forms; but not being able to ascertain this, I have not
coloured them, nor _Durour_ Island, which is described by Carteret as
low.
The CAROLINE ARCHIPELAGO is now well-known, chiefly from the
hydrographical labours of Lutké; it contains about forty groups of
atolls, and three encircled islands, two of which are engraved in
Figures 2 and 7, Plate I. Commencing with the eastern part; the
encircling reef round _Ualen_ appears to be only about half a mile from
the shore; but as the land is low and covered with mangroves (“Voyage
autour du Monde,” par F. Lutké, volume i., page 339), the real margin
has not probably been ascertained. The extreme depth in one of the
harbours within the reef is thirty-three fathoms (see charts in “Atlas
of ‘Coquille’s’ Voyage”), and outside at half a mile distant from the
reef, no bottom was obtained with two hundred and fifty fathoms. The
reef is surmounted by many islets, and the lagoon-like channel within
is mostly shallow, and appears to have been much encroached on by the
low land surrounding the central mountains; these facts show that time
has allowed much detritus to accumulate; coloured pale blue.—
_Pouynipète_, or _Seniavine_. In the greater part of the circumference
of this island, the reef is about one mile and three quarters distant;
on the north side it is five miles off the included high islets. The
reef is broken in several places; and just within it, the depth in one
place is thirty fathoms, and in another, twenty-eight, beyond which, to
all appearance, there was “un porte vaste et sur” (Lutké, volume ii.,
page 4); coloured pale blue.—_Hogoleu_ or _Roug_. This wonderful group
contains at least sixty-two islands, and its reef is one hundred and
thirty-five miles in circuit. Of the islands, only a few, about six or
eight (see “Hydrog. Descrip.” page 428, of the “Voyage of the
‘Astrolabe’,” and the large accompanying chart taken chiefly from that
given by Duperrey) are high, and the rest are all small, low, and
formed on the reef. The depth of the great interior lake has not been
ascertained; but Captain D’Urville appears to have entertained no doubt
about the possibility of taking in a frigate. The reef lies no less
than fourteen miles distant from the northern coasts of the interior
high islands, seven from their western sides, and twenty from the
southern; the sea is deep outside. This island is a likeness on a grand
scale to the Gambier group in the Low Archipelago. Of the groups of
low[1] islands forming the chief part of the Caroline Archipelago, all
those of larger size, have the true atoll-structure (as may be seen in
the “Atlas” by Captain Lutké), and some even of the very small ones, as
_Macaskill_ and _Duperrey_, of which plans are given in the “Atlas of
the ‘Coquille’s’ Voyage.” There are, however, some low small islands of
coral-formation, namely _Ollap_, _Tamatam_, _Bigali_, _Satahoual_,
which do not contain lagoons; but it is probable that lagoons
originally existed, but have since filled up: Lutké (volume ii., page
304) seems to have thought that all the low islands, with only one
exception, contained lagoons. From the sketches, and from the manner in
which the margins of these islands are engraved in the “Atlas of the
Voyage of the ‘Coquille’,” it might have been thought that they were
not low; but by a comparison with the remarks of Lutké (volume ii.,
page 107, regarding Bigali) and of Freycinet (“Hydrog. Memoir
‘L’Uranie’ Voyage,” page 188, regarding Tamatam, Ollap, etc.), it will
be seen that the artist must have represented the land incorrectly. The
most southern island in the group, namely _Piguiram_, is not coloured,
because I have found no account of it. _Nougouor_, or _Monte Verdison_,
which was not visited by Lutké, is described and figured by Mr. Bennett
(“United Service Journal,” January 1832) as an atoll. All the
above-mentioned islands have been coloured blue.
[1] In D’Urville and Lottin’s chart, Peserare is written with capital
letters; but this evidently is an error, for it is one of the low
islets on the reef of Namonouyto (see Lutké’s charts)—a regular atoll.
WESTERN PART OF THE CAROLINE ARCHIPELAGO.—_Fais_ Island is ninety feet
high, and is surrounded, as I have been informed by Admiral Lutké, by a
narrow reef of living coral, of which the broadest part, as represented
in the charts, is only 150 yards; coloured red.— _Philip_ Island., I
believe, is low; but Hunter, in his “Historical Journal,” gives no
clear account of it; uncoloured.—_Elivi;_ from the manner in which the
islets on the reefs are engraved, in the “Atlas of the ‘Astrolabe’s’
Voyage,” I should have thought they were above the ordinary height, but
Admiral Lutké assures me this is not the case: they form a regular
atoll; coloured blue.—_Gouap_ (_Eap_ of Chamisso), is a high island
with a reef (see chart in “Voyage of the ‘Astrolabe’”), more than a
mile distant in most parts from the shore, and two miles in one part.
Captain D’Urville thinks that there would be anchorage (“Hydrog.
Descript. ‘Astrolabe’ Voyage,” page 436) for ships within the reef, if
a passage could be found; coloured pale blue.—_Goulou_, from the chart
in the “‘Astrolabe’s’ Atlas,” appears to be an atoll. D’Urville
(“Hydrog. Descript.” page 437) speaks of the low islets on the reef;
coloured dark blue.
PELEW ISLANDS.—Krusenstern speaks of some of the islands being
mountainous; the reefs are distant from the shore, and there are spaces
within them, and not opposite valleys, with from ten to fifteen
fathoms. According to a MS. chart of the group by Lieutenant Elmer in
the Admiralty, there is a large space within the reef with deepish
water; although the high land does not hold a central position with
respect to the reefs, as is generally the case, I have little doubt
that the reefs of the Pelew Islands ought to be ranked with the barrier
class, and I have coloured them pale blue. In Lieutenant Elmer’s chart
there is a horseshoe-formed shoal, laid down thirteen miles N.W. of
Pelew, with fifteen fathoms within the reef, and some dry banks on it;
coloured dark blue.—_Spanish_, _Martires_, _Sanserot_, _Pulo Anna_ and
_Mariere_ Islands are not coloured, because I know nothing about them,
excepting that according to Krusenstern, the second, third, and fourth
mentioned, are low, placed on coral-reefs, and therefore, perhaps,
contain lagoons; but Pulo Mariere is a little higher.
MARIANA ARCHIPELAGO, or LADRONES.—_Guahan_. Almost the whole of this
island is fringed by reefs, which extend in most parts about a third of
a mile from the land. Even where the reefs are most extensive, the
water within them is shallow. In several parts there is a navigable
channel for boats and canoes within the reefs. In Freycinet’s “Hydrog.
Mem.” there is an account of these reefs, and in the “Atlas,” a map on
a large scale; coloured red.—_Rota_. “L’ile est presque entièrement
entourée des récifs” (page 212, Freycinet’s “Hydrog. Mem.”). These
reefs project about a quarter of a mile from the shore; coloured
red.—_Tinian_. The eastern coast is precipitous, and is without reefs;
but the western side is fringed like the last island; coloured
red.—_Saypan_. The N.E. coast, and likewise the western shores appear
to be fringed; but there is a great, irregular, horn-like reef
projecting far from this side; coloured red.—_Farallon de Medinilla_,
appears so regularly and closely fringed in Freycinet’s charts, that I
have ventured to colour it red, although nothing is said about reefs in
the “Hydrographical Memoir.” The several islands which form the
northern part of the group are volcanic (with the exception perhaps of
Torres, which resembles in form the madreporitic island of Medinilla),
and appear to be without reefs.—_Mangs_, however, is described (by
Freycinet, page 219, “Hydrog.”) from some Spanish charts, as formed of
small islands placed “au milieu des nombreux récifs;” and as these
reefs in the general chart of the group do not project so much as a
mile; and as there is no appearance from a double line, of the
existence of deep water within, I have ventured, although with much
hesitation, to colour them red. Respecting _Folger_ and _Marshall_
Islands which lie some way east of the Marianas, I can find out
nothing, excepting that they are probably low. Krusenstern says this of
Marshall Island; and Folger Island is written with small letters in
D’Urville’s chart; uncoloured.
BONIN or ARZOBISPO GROUP.—_Peel Island_ has been examined by Captain
Beechey, to whose kindness I am much indebted for giving me information
regarding it: “At Port Lloyd there is a great deal of coral; and the
inner harbour is entirely formed by coral-reefs, which extend outside
the port along the coast.” Captain Beechey, in another part of his
letter to me, alludes to the reefs fringing the island in all
directions; but at the same time it must be observed that the surf
washes the volcanic rocks of the coast in the greater part of its
circumference. I do not know whether the other islands of the
Archipelago are fringed; I have coloured Peel Island red.—_Grampus
Island_ to the eastward, does not appear (Meare’s “Voyage,” page 95) to
have any reefs, nor does _Rosario Island_ (from Lutké’s chart), which
lies to the westward. Respecting the few other islands in this part of
the sea, namely the _Sulphur Islands_, with an active volcano, and
those lying between Bonin and Japan (which are situated near the
extreme limit in latitude, at which reefs are formed), I have not been
able to find any clear account.
WEST END OF NEW GUINEA.—_Port Dory_. From the charts in the “Voyage of
the ‘Coquille’,” it would appear that the coast in this part is fringed
by coral-reefs; M. Lesson, however, remarks that the coral is sickly;
coloured red.—_Waigiou_. A considerable portion of the northern shores
of these islands is seen in the charts (on a large scale) in
Freycinet’s “Atlas” to be fringed by coral-reefs. Forrest (page 21,
“Voyage to New Guinea”) alludes to the coral-reefs lining the heads of
Piapis Bay; and Horsburgh (volume ii., page 599, 4th edition), speaking
of the islands in Dampier Strait, says “sharp coral-rocks line their
shores;” coloured red.—In the sea north of these islands, we have
_Guedes_ (or _Freewill_, or _St. David’s_), which from the chart given
in the 4to edition of Carteret’s “Voyage,” must be an atoll.
Krusenstern says the islets are very low; coloured blue.—_Carteret’s
Shoals_, in 2° 53′ N., are described as circular, with stony points
showing all round, with deeper water in the middle; coloured
blue.—_Aiou;_ the plan of this group, given in the “Atlas of the Voyage
of the ‘Astrolabe’,” shows that it is an atoll; and, from a chart in
Forrest’s “Voyage,” it appears that there is twelve fathoms within the
circular reef; coloured blue.—The S.W. coast of New Guinea appears to
be low, muddy, and devoid of reefs. The _Arru_, _Timor laut_, and
_Tenimber_ groups have lately been examined by Captain Kolff, the MS.
translation of which, by Mr. W. Earl, I have been permitted to read,
through the kindness of Captain Washington, R.N. These islands are
mostly rather low, and are surrounded by distant reefs (the Ki Islands,
however, are lofty, and, from Mr. Stanley’s survey, appear without
reefs); the sea in some parts is shallow, in others profoundly deep (as
near Larrat). From the imperfection of the published charts, I have
been unable to decide to which class these reefs belong. From the
distance to which they extend from the land, where the sea is very
deep, I am strongly inclined to believe they ought to come within the
barrier class, and be coloured blue; but I have been forced to leave
them uncoloured.—The last-mentioned groups are connected with the east
end of Ceram by a chain of small islands, of which the small groups of
_Ceram-laut_, _Goram_ and _Keffing_ are surrounded by very extensive
reefs, projecting into deep water, which, as in the last case, I
strongly suspect belong to the barrier class; but I have not coloured
them. From the south side of Keffing, the reefs project five miles
(Windsor Earl’s “Sailing Direct. for the Arafura Sea,” page 9).
CERAM.—In various charts which I have examined, several parts of the
coast are represented as fringed by reefs.—_Manipa_ Island, between
Ceram and Bourou, in an old MS. chart in the Admiralty, is fringed by a
very irregular reef, partly dry at low water, which I do not doubt is
of coral-formation; both islands coloured red.—_Bourou;_ parts of this
island appear fringed by coral-reefs, namely, the eastern coast, as
seen in Freycinet’s chart; and _Cajeli Bay_, which is said by Horsburgh
(volume ii., page 630) to be lined by coral-reefs, that stretch out a
little way, and have only a few feet water on them. In several charts,
portions of the islands forming the AMBOINA GROUP are fringed by reefs;
for instance, _Noessa_, _Harenca_, and _Ucaster_, in Freycinet’s
charts. The above-mentioned islands have been coloured red, although
the evidence is not very satisfactory.—North of Bourou the parallel
line of the _Xulla Isles_ extends: I have not been able to find out
anything about them, excepting that Horsburgh (volume ii., page 543)
says that the northern shore is surrounded by a reef at the distance of
two or three miles; uncoloured.—_Mysol Group;_ the Kanary Islands are
said by Forrest (“Voyage,” page 130) to be divided from each other by
deep straits, and are lined with coral-rocks; coloured red.—_Guebe_,
lying between Waigiou and Gilolo, is engraved as if fringed; and it is
said by Freycinet, that all the soundings under five fathoms were on
coral; coloured red.—_Gilolo_. In a chart published by Dalrymple, the
numerous islands on the western, southern (_Batchian_ and the _Strait
of Patientia_), and eastern sides appear fringed by narrow reefs; these
reefs, I suppose, are of coral, for it is said in “Malte Brun” (volume
xii., page 156), “Sur les cotes (of Batchian) comme _dans le plupart_
des iles de cet archipel, il y a de rocs de medrepores d’une beauté et
d’une variété infinies.” Forrest, also (page 50), says Seland, near
Batchian, is a little island with reefs of coral; coloured red.—_Morty_
Island (north of Gilolo). Horsburgh (volume ii., page 506) says the
northern coast is lined by reefs, projecting one or two miles, and
having no soundings close to them; I have left it uncoloured, although,
as in some former cases, it ought probably to be pale blue.—_Celebes_.
The western and northern coasts appear in the charts to be bold and
without reefs. Near the extreme northern point, however, an islet in
the _Straits of Limbe_, and parts of the adjoining shore, appear to be
fringed: the east side of the bay of _Manado_, has deep water, and is
fringed by sand and coral (“‘Astrol.’ Voyage,” Hydrog. Part, pages
453-4); this extreme point, therefore, I have coloured red.—Of the
islands leading from this point to Magindanao, I have not been able to
find any account, except of _Serangani_, which appears surrounded by
narrow reefs; and Forrest (“Voyage,” page 164) speaks of coral on its
shores; I have, therefore, coloured this island red. To the eastward of
this chain lie several islands; of which I cannot find any account,
except of _Karkalang_, which is said by Horsburgh (volume ii., page
504) to be lined by a dangerous reef, projecting several miles from the
northern shore; not coloured.
ISLANDS NEAR TIMOR.—The account of the following islands is taken from
Captain D. Kolff’s “Voyage,” in 1825, translated by Mr. W. Earl, from
the Dutch.—_Lette_ has “reefs extending along shore at the distance of
half a mile from the land.”—_Moa_ has reefs on the S.W. part.—_Lakor_
has a reef lining its shore; these islands are coloured red.—Still more
eastward, _Luan_ has, differently from the last-mentioned islands, an
extensive reef; it is steep outside, and within there is a depth of
twelve feet; from these facts, it is impossible to decide to which
class this island belongs.—_Kissa_, off the point of Timor, has its
“shore fronted by a reef, steep too on the outer side, over which small
proahs can go at the time of high water;” coloured red.—_Timor;_ most
of the points, and some considerable spaces of the northern shore, are
seen in Freycinet’s chart to be fringed by coral-reefs; and mention is
made of them in the accompanying “Hydrog. Memoir;” coloured
red.—_Savu_, S.E. of Timor, appears in Flinders’ chart to be fringed;
but I have not coloured it, as I do not know that the reefs are of
coral.— _Sandalwood_ Island has, according to Horsburgh (volume ii.,
page 607), a reef on its southern shore, four miles distant from the
land; as the neighbouring sea is deep, and generally bold, this
probably is a barrier- reef, but I have not ventured to colour it.
N.W. COAST OF AUSTRALIA.—It appears, in Captain King’s Sailing
Directions (“Narrative of Survey,” volume ii, pages 325-369), that
there are many extensive coral-reefs skirting, often at considerable
distances, the N.W. shores, and encompassing the small adjoining
islets. Deep water, in no instance, is represented in the charts
between these reefs and the land; and, therefore, they probably belong
to the fringing class. But as they extend far into the sea, which is
generally shallow, even in places where the land seems to be somewhat
precipitous; I have not coloured them. Houtman’s Abrolhos (latitude 28°
S. on west coast) have lately been surveyed by Captain Wickham (as
described in “Naut. Mag.” 1841, page 511): they lie on the edge of a
steeply shelving bank, which extends about thirty miles seaward, along
the whole line of coast. The two southern reefs, or islands, enclose a
lagoon-like space of water, varying in depth from five to fifteen
fathoms, and in one spot with twenty-three fathoms. The greater part of
the island has been formed on their inland sides, by the accumulation
of fragments of coral; the seaward face consisting of nearly bare
ledges of rock. Some of the specimens, brought home by Captain Wickham,
contained fragments of marine shells, but others did not; and these
closely resembled a formation at King George’s Sound, principally due
to the action of the wind on calcareous dust, which I shall describe in
a forthcoming part. From the extreme irregularity of these reefs with
their lagoons, and from their position on a bank, the usual depth of
which is only thirty fathoms, I have not ventured to class them with
atolls, and hence have left them uncoloured.—_Rowley Shoals_. These lie
some way from the N.W. coast of Australia: according to Captain King
(“Narrative of Survey,” volume i., page 60), they are of
coral-formation. They rise abruptly from the sea, and Captain King had
no bottom with 170 fathoms close to them. Three of them are
crescent-shaped; they are mentioned by Mr. Lyell, on the authority of
Captain King, with reference to the direction of their open sides. “A
third oval reef of the same group is entirely submerged” (“Principles
of Geology,” book iii. chapter xviii.); coloured blue.—_Scott’s Reefs_,
lying north of Rowley Shoals, are briefly described by Captain Wickham
(“Naut. Mag.” 1841, page 440): they appear to be of great size, of a
circular form, and “with smooth water within, forming probably a lagoon
of great extent.” There is a break on the western side, where there
probably is an entrance: the water is very deep off these reefs;
coloured blue.
Proceeding westward along the great volcanic chain of the East Indian
Archipelago, _Solor Strait_ is represented in a chart published by
Dalrymple from a Dutch MS., as fringed; as are parts of _Flores_, of
_Adenara_, and of _Solor_. Horsburgh speaks of coral growing on these
shores; and therefore I have no doubt that the reefs are of coral, and
accordingly have coloured them red. We hear from Horsburgh (volume ii.,
page 602) that a coral-flat bounds the shores of _Sapy_ Bay. From the
same authority it appears (page 610) that reefs fringe the island of
_Timor-Young_, on the N. shore of Sumbawa; and, likewise (page 600),
that _Bally_ town in _Lombock_, is fronted by a reef, stretching along
the shore at a distance of a hundred fathoms, with channels through it
for boats; these places, therefore, have been coloured red.—_Bally_
Island. In a Dutch MS. chart on a large scale of Java, which was
brought from that island by Dr. Horsfield, who had the kindness to show
it me at the India House, its western, northern, and southern shores
appear very regularly fringed by a reef (see also Horsburgh, volume
ii., page 593); and as coral is found abundantly there, I have not the
least doubt that the reef is of coral, and therefore have coloured it
red.
JAVA.—My information regarding the reefs of this great island is
derived from the chart just mentioned. The greater part of _Madura_ is
represented in it as regularly fringed, and likewise portions of the
coast of Java immediately south of it. Dr. Horsfield informs me that
coral is very abundant near _Sourabaya_. The islets and parts of the N.
coast of Java, west of _Point Buang_, or _Japara_, are fringed by
reefs, said to be of coral. _Lubeck_, or _Bavian_ Islands, lying at
some distance from the shore of Java, are regularly fringed by
coral-reefs. _Carimon Java_ appears equally so, though it is not
directly said that the reefs are of coral; there is a depth between
thirty and forty fathoms round these islands. Parts of the shores of
_Sunda_ Strait, where the water is from forty to eighty fathoms deep,
and the islets near _Batavia_ appear in several charts to be fringed.
In the Dutch chart the southern shore, in the narrowest part of the
island, is in two places fringed by reefs of coral. West of
_Segorrowodee_ Bay, and the extreme S.E. and E. portions are likewise
fringed by coral-reefs; all the above-mentioned places coloured red.
_Macassar Strait_; the east coast of _Borneo_ appears, in most parts,
free from reefs, and where they occur, as on the east coast of
_Pamaroong_, the sea is very shallow; hence no part is coloured. In
_Macassar_ Strait itself, in about latitude 2° S., there are many small
islands with coral-shoals projecting far from them. There are also (old
charts by Dalrymple) numerous little flats of coral, not rising to the
surface of the water, and shelving suddenly from five fathoms to no
bottom with fifty fathoms; they do not appear to have a lagoon-like
structure. There are similar coral-shoals a little farther south; and
in latitude 4° 55′ there are two, which are engraved from modern
surveys, in a manner which might represent an annular reef with deep
water inside: Captain Moresby, however, who was formerly in this sea,
doubts this fact, so that I have left them uncoloured: at the same time
I may remark, that these two shoals make a nearer approach to the
atoll-like structure than any other within the E. Indian Archipelago.
Southward of these shoals there are other low islands and irregular
coral-reefs; and in the space of sea, north of the great volcanic
chain, from Timor to Java, we have also other islands, such as the
_Postillions_, _Kalatoa_, _Tokan-Bessees_, &c., which are chiefly low,
and are surrounded by very irregular and distant reefs. From the
imperfect charts I have seen, I have not been able to decide whether
they belong to the atoll or barrier-classes, or whether they merely
fringe submarine banks, and gently sloping land. In the Bay of _Bonin_,
between the two southern arms of Celebes, there are numerous coral-
reefs; but none of them seem to have an atoll-like structure. I have,
therefore, not coloured any of the islands in this part of the sea; I
think it, however, exceedingly probable that some of them ought to be
blue. I may add that there is a harbour on the S.E. coast of _Bouton_
which, according to an old chart, is formed by a reef, parallel to the
shore, with deep water within; and in the “Voyage of the ‘Coquille’,”
some neighbouring islands are represented with reefs a good way
distant, but I do not know whether with deep water within. I have not
thought the evidence sufficient to permit me to colour them.
SUMATRA.—Commencing with the west coast and outlying islands, _Engano
Island_ is represented in the published chart as surrounded by a narrow
reef, and Napier, in his “Sailing Directions,” speaks of the reef being
of coral (also Horsburgh, volume ii., page 115); coloured red.—_Rat
Island_ (3° 51′ S.) is surrounded by reefs of coral, partly dry at low
water, (Horsburgh, volume ii., page 96).—_Trieste Island_ (4° 2′ S.).
The shore is represented in a chart which I saw at the India House, as
fringed in such a manner, that I feel sure the fringe consists of
coral; but as the island is so low, that the sea sometimes flows quite
over it (Dampier, “Voyage,” volume i., page 474), I have not coloured
it.—_Pulo Dooa_ (latitude 3°). In an old chart it is said there are
chasms in the reefs round the island, admitting boats to the
watering-place, and that the southern islet consists of a mass of sand
and coral.—_Pulo Pisang;_ Horsburgh (volume ii., page 86) says that the
rocky coral-bank, which stretches about forty yards from the shore, is
steep to all round: in a chart, also, which I have seen, the island is
represented as regularly fringed.—_Pulo Mintao_ is lined with reefs on
its west side (Horsburgh, volume ii., page 107).—_Pulo Baniak;_ the
same authority (volume ii., page 105), speaking of a part, says it is
faced with coral-rocks.—_Minguin_ (3° 36′ N.). A coral-reef fronts this
place, and projects into the sea nearly a quarter of a mile (“Notices
of the Indian Arch.” published at Singapore, page 105).—_Pulo Brassa_
(5° 46′ N.). A reef surrounds it at a cable’s length (Horsburgh, volume
ii., page 60). I have coloured all the above-specified points red. I
may here add, that both Horsburgh and Mr. Moor (in the “Notices” just
alluded to) frequently speak of the numerous reefs and banks of coral
on the west coast of Sumatra; but these nowhere have the structure of a
barrier-reef, and Marsden (“History of Sumatra”) states, that where the
coast is flat, the fringing-reefs extend furthest from it. The northern
and southern points, and the greater part of the east coast, are low,
and faced with mud banks, and therefore without coral.
NICOBAR ISLANDS.—The chart represents the islands of this group as
fringed by reefs. With regard to _Great Nicobar_, Captain Moresby
informs me, that it is fringed by reefs of coral, extending between two
and three hundred yards from the shore. The _Northern Nicobars_ appear
so regularly fringed in the published charts, that I have no doubt the
reefs are of coral. This group, therefore, is coloured red.
ANDAMAN ISLANDS.—From an examination of the MS. chart, on a large
scale, of this island, by Captain Arch. Blair, in the Admiralty,
several portions of the coast appear fringed; and as Horsburgh speaks
of coral-reefs being numerous in the vicinity of these islands, I
should have coloured them red, had not some expressions in a paper in
the “Asiatic Researches” (volume iv., page 402) led me to doubt the
existence of reefs; uncoloured.
The coast of _Malacca_, _Tenasserim_ and the coasts northward, appear
in the greater part to be low and muddy: where reefs occur, as in parts
of _Malacca Straits_, and near _Singapore_, they are of the fringing
kind; but the water is so shoal, that I have not coloured them. In the
sea, however, between Malacca and the west coast of Borneo, where there
is a greater depth from forty to fifty fathoms, I have coloured red
some of the groups, which are regularly fringed. The northern _Natunas_
and the _Anambas_ Islands are represented in the charts on a large
scale, published in the “Atlas of the Voyage of the ‘Favourite’,” as
fringed by reefs of coral, with very shoal water within
them.—_Tumbelan_ and _Bunoa_ Islands (1° N.) are represented in the
English charts as surrounded by a very regular fringe.— _St. Barbes_
(0° 15′ N.) is said by Horsburgh (volume ii., page 279) to be fronted
by a reef, over which boats can land only at high water.—The shore of
_Borneo_ at _Tunjong Apee_ is also fronted by a reef, extending not far
from the land (Horsburgh, volume ii., page 468). These places I have
coloured red; although with some hesitation, as the water is shallow. I
might perhaps have added _Pulo Leat_, in Gaspar Strait, _Lucepara_, and
_Carimata;_ but as the sea is confined and shallow, and the reefs not
very regular, I have left them uncoloured.
The water shoals gradually towards the whole west coast of _Borneo:_ I
cannot make out that it has any reefs of coral. The islands, however,
off the northern extremity, and near the S.W. end of _Palawan_, are
fringed by very distant coral-reefs; thus the reefs in the case of
_Balabac_ are no less than five miles from the land; but the sea, in
the whole of this district, is so shallow, that the reefs might be
expected to extend very far from the land. I have not, therefore,
thought myself authorised to colour them. The N.E. point of Borneo,
where the water is very shoal, is connected with Magindanao by a chain
of islands called the _Sooloo Archipelago_, about which I have been
able to obtain very little information; _Pangootaran_, although ten
miles long, entirely consists of a bed of coral-rock (“Notices of E.
Indian Arch.” page 58): I believe from Horsburgh that the island is
low; not coloured.—_Tahow Bank_, in some old charts, appears like a
submerged atoll; not coloured. Forrest (“Voyage,” page 21) states that
one of the islands near Sooloo is surrounded by coral-rocks; but there
is no distant reef. Near the S. end of _Basselan_, some of the islets
in the chart accompanying Forrest’s “Voyage,” appear fringed with
reefs; hence I have coloured, though unwillingly, parts of the Sooloo
group red. The sea between Sooloo and Palawan, near the shoal coast of
Borneo, is interspersed with irregular reefs and shoal patches; not
coloured: but in the northern part of this sea, there are two low
islets, _Cagayanes_ and _Cavilli_, surrounded by extensive coral-reefs;
the breakers round the latter (Horsburgh, volume ii., page 513) extend
five or six miles from a sandbank, which forms the only dry part; these
breakers are steep to outside; there appears to be an opening through
them on one side, with four or five fathoms within: from this
description, I strongly suspect that Cavilli ought to be considered an
atoll; but, as I have not seen any chart of it, on even a moderately
large scale, I have not coloured it. The islets off the northern end of
_Palawan_, are in the same case as those off the southern end, namely
they are fringed by reefs, some way distant from the shore, but the
water is exceedingly shallow; uncoloured. The western shore of Palawan
will be treated of under the head of China Sea.
PHILIPPINE ARCHIPELAGO.—A chart on a large scale of _Appoo shoal_,
which lies near the S.E. coast of Mindoro, has been executed by Captain
D. Ross: it appears atoll-formed, but with rather an irregular outline;
its diameter is about ten miles; there are two well-defined passages
leading into the interior lagoon, which appears open; close outside the
reef all round, there is no bottom with seventy fathoms; coloured
blue.—_Mindoro:_ the N.W. coast is represented in several charts, as
fringed by a reef, and _Luban Island_ is said, by Horsburgh (volume
ii., page 436), to be “lined by a reef.”—_Luzon:_ Mr. Cuming, who has
lately investigated with so much success the Natural History of the
Philippines, informs me, that about three miles of the shore north of
Point St. Jago, is fringed by a reef; as are (Horsburgh, volume ii.,
page 437) the Three Friars off Silanguin Bay. Between Point Capones and
Playa Honda, the coast is “lined by a coral-reef, stretching out nearly
a mile in some places,” (Horsburgh); and Mr. Cuming visited some
fringing- reefs on parts of this coast, namely, near Puebla, Iba, and
Mansinglor. In the neighbourhood of Solon-solon Bay, the shore is lined
(Horsburgh ii., page 439) by coral-reefs, stretching out a great way:
there are also reefs about the islets off Solamague; and as I am
informed by Mr. Cuming, near St. Catalina, and a little north of it.
The same gentleman informs me there are reefs on the S.E. point of this
island in front of Samar, extending from Malalabon to Bulusan. These
appear to be the principal fringing-reefs on the coasts of Luzon; and
they have all been coloured red. Mr. Cuming informs me that none of
them have deep water within; although it appears from Horsburgh that
some few extend to a considerable distance from the shore. Within the
Philippine Archipelago, the shores of the islands do not appear to be
commonly fringed, with the exception of the S. shore of _Masbate_, and
nearly the whole of _Bohol;_ which are both coloured red. On the S.
shore of _Magindanao_, Bunwoot Island is surrounded (according to
Forrest, “Voyage,” page 253), by a coral-reef, which in the chart
appears one of the fringing class. With respect to the eastern coasts
of the whole Archipelago, I have not been able to obtain any account.
BABUYAN ISLANDS.—Horsburgh says (volume ii., page 442), coral-reefs
line the shores of the harbour in Fuga; and the charts show there are
other reefs about these islands. _Camiguin_ has its shore in parts
lined by coral-rock (Horsburgh, page 443); about a mile off shore there
is between thirty and thirty-five fathoms. The plan of Port San Pio
Quinto shows that its shores are fringed with coral; coloured
red.—BABUYAN ISLANDS: Horsburgh, speaking of the southern part of the
group (volume ii., page 445) says the shores of both islands are
fortified by a reef, and through some of the gaps in it, the natives
can pass in their boats in fine weather; the bottom near the land is
coral-rock. From the published charts, it is evident that several of
these islands are most regularly fringed; coloured red. The northern
islands are left uncoloured, as I have been unable to find any account
of them.—FORMOSA. The shores, especially the western one, seem chiefly
composed of mud and sand, and I cannot make out that they are anywhere
lined by reefs; except in a harbour (Horsburgh, volume ii., page 449)
at the extreme northern point: hence, of course, the whole of this
island is left uncoloured. The small adjoining islands are in the same
case.— PATCHOW, or MADJIKO-SIMA GROUPS. _Patchuson_ has been described
by Captain Broughton (“Voy. to the N. Pacific,” page 191); he says, the
boats, with some difficulty, found a passage through the coral-reefs,
which extend along the coast, nearly half a mile off it. The boats were
well sheltered within the reef; but it does not appear that the water
is deep there. Outside the reef the depth is very irregular, varying
from five to fifty fathoms; the form of the land is not very abrupt;
coloured red.—_Taypin-san;_ from the description given (page 195) by
the same author, it appears that a very irregular reef extends, to the
distance of several miles, from the southern island; but whether it
encircles a space of deep water is not evident; nor, indeed, whether
these outlying reefs are connected with those more immediately
adjoining the land; left uncoloured. I may here just add that the shore
of _Kumi_ (lying west of Patchow), has a narrow reef attached to it in
the plan of it, in La Peyrouse’s “Atlas;” but it does not appear in the
account of the voyage that it is of coral; uncoloured.—LOO CHOO. The
greater part of the coast of this moderately hilly island, is skirted
by reefs, which do not extend far from the shore, and which do not
leave a channel of deep water within them, as may be seen in the charts
accompanying Captain B. Hall’s voyage to Loo Choo (see also remarks in
Appendix, pages xxi. and xxv.). There are, however, some ports with
deep water, formed by reefs in front of valleys, in the same manner as
happens at Mauritius. Captain Beechey, in a letter to me, compares
these reefs with those encircling the Society Islands; but there
appears to me a marked difference between them, in the less distance at
which the Loo Choo reefs lie from the land with relation to the
probable submarine inclination, and in the absence of an interior deep
water-moat or channel, parallel to the land. Hence, I have classed
these reefs with fringing-reefs, and coloured them red.—PESCADORES
(west of Formosa). Dampier (volume i., page 416), has compared the
appearance of the land to the southern parts of England. The islands
are interlaced with coral-reefs; but as the water is very shoal, and as
spits of sand and gravel (Horsburgh, volume ii., page 450) extend far
out from them, it is impossible to draw any inferences regarding the
nature of the reefs.
CHINA SEA.—Proceeding from north to south, we first meet the _Pratas
Shoal_ (latitude 20° N.) which, according to Horsburgh (volume ii.,
page 335), is composed of coral, is of a circular form, and has a low
islet on it. The reef is on a level with the water’s edge, and when the
sea runs high, there are breakers mostly all round, “but the water
within seems pretty deep in some places; although steep-to in most
parts outside, there appear to be several parts where a ship might find
anchorage outside the breakers;” coloured blue.—The _Paracells_ have
been accurately surveyed by Captain D. Ross, and charts on a large
scale published: but few low islets have been formed on these shoals,
and this seems to be a general circumstance in the China Sea; the sea
close outside the reefs is very deep; several of them have a
lagoon-like structure; or separate islets (_Prattle_, _Robert_,
_Drummond_, etc.) are so arranged round a moderately shallow space, as
to appear as if they had once formed one large atoll.— _Bombay Shoal_
(one of the Paracells) has the form of an annular reef, and is
“apparently deep within;” it seems to have an entrance (Horsburgh,
volume ii., page 332) on its west side; it is very steep
outside.—_Discovery Shoal_, also is of an oval form, with a lagoon-like
space within, and three openings leading into it, in which there is a
depth from two to twenty fathoms. Outside, at the distance (Horsburgh,
volume ii., page 333) of only twenty yards from the reef, soundings
could not be obtained. The Paracells are coloured blue.—_Macclesfield
Bank:_ this is a coral-bank of great size, lying east of the Paracells;
some parts of the bank are level, with a sandy bottom, but, generally,
the depth is very irregular. It is intersected by deep cuts or
channels. I am not able to perceive in the published charts (its
limits, however, are not very accurately known) whether the central
part is deeper, which I suspect is the case, as in the Great Chagos
Bank, in the Indian Ocean; not coloured.—_Scarborough Shoal:_ this
coral-shoal is engraved with a double row of crosses, forming a circle,
as if there was deep water within the reef: close outside there was no
bottom, with a hundred fathoms; coloured blue.—The sea off the west
coast of Palawan and the northern part of Borneo is strewed with
shoals: _Swallow Shoal_, according to Horsburgh (volume ii., page 431)
“is formed, _like most_ of the shoals hereabouts, of a belt of
coral-rocks, “with a basin of deep water within.”—_Half-Moon Shoal_ has
a similar structure; Captain D. Ross describes it, as a narrow belt of
coral-rock, “with a basin of deep water in the centre,” and deep sea
close outside.—_Bombay Shoal_ appears (Horsburgh, volume ii., page 432)
“to be a basin of smooth water surrounded by breakers.” These three
shoals I have coloured blue.—The _Paraquas Shoals_ are of a circular
form, with deep gaps running through them; not coloured.—A bank
gradually shoaling to the depth of thirty fathoms, extends to a
distance of about twenty miles from the northern part of _Borneo_, and
to thirty miles from the northern part of _Palawan_. Near the land this
bank appears tolerably free from danger, but a little further out it is
thickly studded with coral-shoals, which do not generally rise quite to
the surface; some of them are very steep-to, and others have a fringe
of shoal-water round them. I should have thought that these shoals had
level surfaces, had it not been for the statement made by Horsburgh
“that most of the shoals hereabouts are formed of a belt of coral.”
But, perhaps that expression was more particularly applied to the
shoals further in the offing. If these reefs of coral have a
lagoon-like structure, they should have been coloured blue, and they
would have formed an imperfect barrier in front of Palawan and the
northern part of Borneo. But, as the water is not very deep, these
reefs may have grown up from inequalities on the bank: I have not
coloured them.—The coast of _China_, _Tonquin_, and _Cochin-China_,
forming the western boundary of the China Sea, appear to be without
reefs: with regard to the two last-mentioned coasts, I speak after
examining the charts on a large scale in the “Atlas of the Voyage of
the ‘Favourite’.”
INDIAN OCEAN.—_South Keeling_ has been specially described: nine miles
north of it lies North Keeling, a very small atoll, surveyed by the
“Beagle,” the lagoon of which is dry at low water.—_Christmas Island_,
lying to the east, is a high island, without, as I have been informed
by a person who passed it, any reefs at all.—CEYLON: a space about
eighty miles in length of the south-western and southern shores of
these islands has been described by Mr. Twynam (“Naut. Mag.” 1836,
pages 365 and 518); parts of this space appear to be very regularly
fringed by coral-reefs, which extend from a quarter to half a mile from
the shore. These reefs are in places breached, and afford safe
anchorage for the small trading craft. Outside, the sea gradually
deepens; there is forty fathoms about six miles off shore: this part I
have coloured red. In the published charts of Ceylon there appear to be
fringing-reefs in several parts of the south-eastern shores, which I
have also coloured red.—At Venloos Bay the shore is likewise fringed.
North of Trincomalee there are also reefs of the same kind. The sea off
the northern part of Ceylon is exceedingly shallow; and therefore I
have not coloured the reefs which fringe portions of its shores, and
the adjoining islets, as well as the Indian promontory of _Madura_.
CHAGOS, MALDIVA, and LACCADIVE ARCHIPELAGOES.—These three great groups
which have already been often noticed, are now well-known from the
admirable surveys of Captain Moresby and Lieutenant Powell. The
published charts, which are worthy of the most attentive examination,
at once show that the _Chagos_ and _Maldiva_ groups are entirely formed
of great atolls, or lagoon-formed reefs, surmounted by islets. In the
_Laccadive_ group, this structure is less evident; the islets are low,
not exceeding the usual height of coral-formations (see Lieutenant
Wood’s account, “Geographical Journal”, volume vi., page 29), and most
of the reefs are circular, as may be seen in the published charts; and
within several of them, as I am informed by Captain Moresby, there is
deepish water; these, therefore, have been coloured blue. Directly
north, and almost forming part of this group, there is a long, narrow,
slightly curved bank, rising out of the depths of the ocean, composed
of sand, shells, and decayed coral, with from twenty-three to thirty
fathoms on it. I have no doubt that it has had the same origin with the
other Laccadive banks; but as it does not deepen towards the centre I
have not coloured it. I might have referred to other authorities
regarding these three archipelagoes; but after the publication of the
charts by Captain Moresby, to whose personal kindness in giving me much
information I am exceedingly indebted, it would have been superfluous.
_Sahia de Malha_bank consists of a series of narrow banks, with from
eight to sixteen fathoms on them; they are arranged in a semicircular
manner, round a space about forty fathoms deep, which slopes on the
S.E. quarter to unfathomable depths; they are steep-to on both sides,
but more especially on the ocean-side. Hence this bank closely
resembles in structure, and I may add from Captain Moresby’s
information in composition, the Pitt’s Bank in the Chagos group; and
the Pitt’s Bank, must, after what has been shown of the Great Chagos
Bank, be considered as a sunken, half-destroyed atoll; hence coloured
blue.—_Cargados Carajos Bank_. Its southern portion consists of a
large, curved, coral-shoal, with some low islets on its eastern edge,
and likewise some on the western side, between which there is a depth
of about twelve fathoms. Northward, a great bank extends. I cannot
(probably owing to the want of perfect charts) refer this reef and bank
to any class;—therefore not coloured.—_Ile de Sable_ is a little
island, lying west of C. Carajos, only some toises in height (“Voyage
of the ‘Favourite’,” volume i., page 130); it is surrounded by reefs;
but its structure is unintelligible to me. There are some small banks
north of it, of which I can find no clear account.—_Mauritius_. The
reefs round this island have been described in the chapter on
fringing-reefs; coloured red. —_Rodriguez_. The coral-reefs here are
exceedingly extensive; in one part they project even five miles from
the shore. As far as I can make out, there is no deep-water moat within
them; and the sea outside does not deepen very suddenly. The outline,
however, of the land appears to be (“Life of Sir J. Makintosh,” volume
ii., page 165) hilly and rugged. I am unable to decide whether these
reefs belong to the barrier class; as seems probable from their great
extension, or to the fringing class; uncoloured. —_Bourbon_. The
greater part of the shores of this island are without reefs; but
Captain Carmichael (Hooker’s “Bot. Misc.”) states that a portion,
fifteen miles in length, on the S.E. side, is imperfectly fringed with
coral reefs: I have not thought this sufficient to colour the island.
SEYCHELLES.—The rocky islands of primary formation, composing this
group, rise from a very extensive and tolerably level bank, having a
depth between twenty and forty fathoms. In Captain Owen’s chart, and in
that in the “Atlas of the Voyage of the ‘Favourite’,” it appears that
the east side of _Mahé_ and the adjoining islands of _St. Anne_ and
_Cerf_, are regularly fringed by coral-reefs. A portion of the S.E.
part of _Curieuse Island_, the N., and part of the S.W. shore of
_Praslin Island_, and the whole west side of _Digue Island_, appear
fringed. From a MS. account of these islands by Captain F. Moresby, in
the Admiralty, it appears that _Silhouette_ is also fringed; he states
that all these islands are formed of granite and quartz, that they rise
abruptly from the sea, and that “coral-reefs have grown round them, and
project for some distance.” Dr. Allan, of Forres, who visited these
islands, informs me that there is no deep water between the reefs and
the shore. The above specified points have been coloured red.
_Amirantes Islands:_ The small islands of this neighbouring group,
according to the MS. account of them by Captain F. Moresby, are
situated on an extensive bank; they consist of the debris of corals and
shells; are only about twenty feet in height, and are environed by
reefs, some attached to the shore, and some rather distant from it.—I
have taken great pains to procure plans and information regarding the
several islands lying between S.E. and S.W. of the Amirantes, and the
Seychelles; relying chiefly on Captain F. Moresby and Dr. Allan, it
appears that the greater number, namely—_Platte_, _Alphonse_,
_Coetivi_, _Galega_, _Providence_, _St. Pierre_, _Astova_,
_Assomption_, and _Glorioso_, are low, formed of sand or coral-rock,
and irregularly shaped; they are situated on very extensive banks, and
are connected with great coral-reefs. Galega is said by Dr. Allan, to
be rather higher than the other islands; and St. Pierre is described by
Captain F. Moresby, as being cavernous throughout, and as not
consisting of either limestone or granite. These islands, as well as
the Amirantes, certainly are not atoll-formed, and they differ as a
group from every other group with which I am acquainted; I have not
coloured them; but probably the reefs belong to the fringing class.
Their formation is attributed, both by Dr. Allan and Captain F.
Moresby, to the action of the currents, here exceedingly violent, on
banks, which no doubt have had an independent geological origin. They
resemble in many respects some islands and banks in the West Indies,
which owe their origin to a similar agency, in conjunction with an
elevation of the entire area. In close vicinity to the several islands,
there are three others of an apparently different nature: first, _Juan
de Nova_, which appears from some plans and accounts to be an atoll;
but from others does not appear to be so; not coloured. Secondly
_Cosmoledo;_ “this group consists of a ring of coral, ten leagues in
circumference, and a quarter of a mile broad in some places, enclosing
a magnificent lagoon, into which there did not appear a single opening”
(Horsburgh, volume i., page 151); coloured blue. Thirdly, _Aldabra;_ it
consists of three islets, about twenty-five feet in height, with red
cliffs (Horsburgh, volume i., page 176) surrounding a very shallow
basin or lagoon. The sea is profoundly deep close to the shore. Viewing
this island in a chart, it would be thought an atoll; but the foregoing
description shows that there is something different in its nature; Dr.
Allan also states that it is cavernous, and that the coral-rock has a
vitrified appearance. Is it an upheaved atoll, or the crater of a
volcano?—uncoloured.
COMORO GROUP.—_Mayotta_, according to Horsburgh (volume i., page 216,
4th edition), is completely surrounded by a reef, which runs at the
distance of three, four, and in some places even five miles from the
land; in an old chart, published by Dalrymple, a depth in many places
of thirty-six and thirty-eight fathoms is laid down within the reef. In
the same chart, the space of open water within the reef in some parts
is even more than three miles wide: the land is bold and peaked; this
island, therefore, is encircled by a well-characterised barrier-reef,
and is coloured pale blue.—_Johanna;_ Horsburgh says (volume I. page
217) this island from the N.W. to the S.W. point, is bounded by a reef,
at the distance of two miles from the shore; in some parts, however,
the reef must be attached, since Lieutenant Boteler (“Narr.” volume i.,
page 161) describes a passage through it, within which there is room
only for a few boats. Its height, as I am informed by Dr. Allan, is
about 3,500 feet; it is very precipitous, and is composed of granite,
greenstone, and quartz; coloured blue.—_Mohilla;_ on the S. side of
this island there is anchorage, in from thirty to forty-five fathoms,
between a reef and the shore (Horsburgh, volume i., page 214); in
Captain Owen’s chart of Madagascar, this island is represented as
encircled; coloured blue.—_Great Comoro Island_ is, as I am informed by
Dr. Allan, about 8,000 feet high, and apparently volcanic; it is not
regularly encircled; but reefs of various shapes and dimensions, jut
out from every headland on the W., S., and S.E. coasts, inside of which
reefs there are channels, often parallel with the shore, with deep
water. On the north-western coasts the reefs appear attached to the
shores. The land near the coast is in some places bold, but generally
speaking it is flat; Horsburgh says (volume i., page 214) the water is
profoundly deep close to the _shore_, from which expression I presume
some parts are without reefs. From this description I apprehend the
reef belongs to the barrier class; but I have not coloured it, as most
of the charts which I have seen, represent the reefs round it as very
much less extensive than round the other islands in the group.
MADAGASCAR.—My information is chiefly derived from the published charts
by Captain Owen, and the accounts given by him and by Lieutenant
Boteler. Commencing at the S.W. extremity of the island; towards the
northern part of the _Star Bank_ (in latitude 25° S.) the coast for ten
miles is fringed by a reef; coloured red. The shore immediately S. of
_St. Augustine’s Bay_ appears fringed; but _Tullear_ Harbour, directly
N. of it, is formed by a narrow reef ten miles long, extending parallel
to the shore, with from four to ten fathoms within it. If this reef had
been more extensive, it must have been classed as a barrier-reef; but
as the line of coast falls inwards here, a submarine bank perhaps
extends parallel to the shore, which has offered a foundation for the
growth of the coral; I have left this part uncoloured. From _lat_. 22°
16′ _to_ 21° 37′, the shore is fringed by coral-reefs (see Lieutenant
Boteler’s “Narrative,” volume ii., page 106), less than a mile in
width, and with shallow water within. There are outlying coral-shoals
in several parts of the offing, with about ten fathoms between them and
the shore, and the depth of the sea one mile and a half seaward, is
about thirty fathoms. The part above specified is engraved on a large
scale; and as in the charts on rather a smaller scale the same fringe
of reef extends as far as latitude 33° 15′; I have coloured the whole
of this part of the coast red. The islands of _Juan de Nova_ (in
latitude 17° S.) appear in the charts on a large scale to be fringed,
but I have not been able to ascertain whether the reefs are of coral;
uncoloured. The main part of the west coast appears to be low, with
outlying sandbanks, which, Lieutenant Boteler (volume ii., page 106)
says, “are faced on the edge of deep water by a line of sharp-pointed
coral-rocks.” Nevertheless I have not coloured this part, as I cannot
make out by the charts that the coast itself is fringed. The headlands
of _Narrenda_ and _Passandava Bays_ (14° 40′) and the islands in front
of _Radama harbour_ are represented in the plans as regularly fringed,
and have accordingly been coloured red. With respect to the _East coast
of Madagascar_, Dr. Allan informs me in a letter, that the whole line
of coast, from _Tamatave_, in 18° 12′, to _C. Amber_, at the extreme
northern point of the island, is bordered by coral-reefs. The land is
low, uneven, and gradually rising from the coast. From Captain Owen’s
charts, also, the existence of these reefs, which evidently belong to
the fringing class, on some parts, namely N. of _British Sound_, and
near _Ngoncy_, of the above line of coast might have been inferred.
Lieutenant Boteler (volume i., page 155) speaks of “the reef
surrounding the island of _St. Mary’s_ at a small distance from the
shore.” In a previous chapter I have described, from the information of
Dr. Allan, the manner in which the reefs extend in N.E. lines from the
headlands on this coast, thus sometimes forming rather deep channels
within them, this seems caused by the action of the currents, and the
reefs spring up from the submarine prolongations of the sandy
headlands. The above specified portion of the coast is coloured red.
The remaining S.E. portions do not appear in any published chart to
possess reefs of any kind; and the Rev. W. Ellis, whose means of
information regarding this side of Madagascar have been extensive,
informs me he believes there are none.
EAST COAST OF AFRICA.—Proceeding from the northern part, the coast
appears, for a considerable space, without reefs. My information, I may
here observe, is derived from the survey by Captain Owen, together with
his narrative; and that by Lieutenant Boteler. At _Mukdeesha_ (10° 1′
N.) there is a coral-reef extending four or five miles along the shore
(Owen’s “Narr.” volume i, page 357) which in the chart lies at the
distance of a quarter of a mile from the shore, and has within it from
six to ten feet water: this then is a fringing-reef, and is coloured
red. From _Juba_, a little S. of the equator, to _Lamoo_ (in 2° 20′ S.)
“the coast and islands are formed of madrepore” (Owen’s “Narrative,”
volume i., page 363). The chart of this part (entitled _Dundas
Islands_), presents an extraordinary appearance; the coast of the
mainland is quite straight and it is fronted at the average distance of
two miles, by exceedingly narrow, straight islets, fringed with reefs.
Within the chain of islets, there are extensive tidal flats and muddy
bays, into which many rivers enter; the depths of these spaces varies
from one to four fathoms—the latter depth not being common, and about
twelve feet the average. Outside the chain of islets, the sea, at the
distance of a mile, varies in depth from eight to fifteen fathoms.
Lieutenant Boteler (“Narr.” volume i., page 369) describes the muddy
bay of _Patta_, which seems to resemble other parts of this coast, as
fronted by small, narrow, level islets formed of decomposing coral, the
margin of which is seldom of greater height than twelve feet,
overhanging the rocky surface from which the islets rise. Knowing that
the islets are formed of coral, it is, I think, scarcely possible to
view the coast, and not at once conclude that we here see a
fringing-reef, which has been upraised a few feet: the unusual depth of
from two to four fathoms within some of these islets, is probably due
to muddy rivers having prevented the growth of coral near the shore.
There is, however, one difficulty on this view, namely, that before the
elevation took place, which converted the reef into a chain of islets,
the water must apparently have been still deeper; on the other hand it
may be supposed that the formation of a nearly perfect barrier in
front, of so large an extent of coast, would cause the currents
(especially in front of the rivers), to deepen their muddy beds. When
describing in the chapter on fringing-reefs, those of Mauritius, I have
given my reasons for believing that the shoal spaces within reefs of
this kind, must, in many instances, have been deepened. However this
may be, as several parts of this line of coast are undoubtedly fringed
by living reefs, I have coloured it red.— _Maleenda_ (3° 20′ S.). In
the plan of the harbour, the south headland appears fringed; and in
Owen’s chart on a larger scale, the reefs are seen to extend nearly
thirty miles southward; coloured red.—_Mombas_ (4° 5′ S.). The island
which forms the harbour, “is surrounded by cliffs of madrepore, capable
of being rendered almost impregnable” (Owen’s “Narr.” volume i., page
412). The shore of the mainland N. and S. of the harbour, is most
regularly fringed by a coral-reef at a distance from half a mile to one
mile and a quarter from the land; within the reef the depth is from
nine to fifteen feet; outside the reef the depth at rather less than
half a mile is thirty fathoms. From the charts it appears that a space
about thirty-six miles in length, is here fringed; coloured
red.—_Pemba_ (5° S.) is an island of coral-formation, level, and about
two hundred feet in height (Owen’s “Narr.” volume i., page 425); it is
thirty-five miles long, and is separated from the mainland by a deep
sea. The outer coast is represented in the chart as regularly fringed;
coloured red. The mainland in front of Pemba is likewise fringed; but
there also appear to be some outlying reefs with deep water between
them and the shore. I do not understand their structure, either from
the charts or the description, therefore have not coloured
them.—_Zanzibar_ resembles Pemba in most respects; its southern half on
the western side and the neighbouring islets are fringed; coloured red.
On the mainland, a little S. of Zanzibar, there are some banks parallel
to the coast, which I should have thought had been formed of coral, had
it not been said (Boteler’s “Narr.” volume ii., page 39) that they were
composed of sand; not coloured.—_Latham’s Bank_ is a small island,
fringed by coral-reefs; but being only ten feet high, it has not been
coloured.—_Monfeea_ is an island of the same character as Pemba; its
outer shore is fringed, and its southern extremity is connected with
Keelwa Point on the mainland by a chain of islands fringed by reefs;
coloured red. The four last-mentioned islands resemble in many respects
some of the islands in the Red Sea, which will presently be described.—
_Keelwa_. In a plan of the shore, a space of twenty miles N. and S. of
this place is fringed by reefs, apparently of coral: these reefs are
prolonged still further southward in Owen’s general chart. The coast in
the plans of the rivers _Lindy_ and _Monghow_ (9° 59′ and 10° 7′ S.)
has the same structure; coloured red.—_Querimba Islands_ (from 10° 40′
to 13° S.). A chart on a large scale is given of these islands; they
are low, and of coral-formation (Boteler’s “Narr.” volume ii., page
54); and generally have extensive reefs projecting from them which are
dry at low water, and which on the outside rise abruptly from a deep
sea: on their insides they are separated from the continent by a
channel, or rather a succession of bays, with an average depth of ten
fathoms. The small headlands on the continent also have coral-banks
attached to them; and the Querimba islands and banks are placed on the
lines of prolongation of these headlands, and are separated from them
by very shallow channels. It is evident that whatever cause, whether
the drifting of sediment or subterranean movements, produced the
headlands, likewise produced, as might have been expected, submarine
prolongations to them; and these towards their outer extremities, have
since afforded a favourable basis for the growth of coral-reefs, and
subsequently for the formation of islets. As these reefs clearly belong
to the fringing class, the Querimba islands have been coloured
red.—_Monabila_ (13° 32′ S.). In the plan of this harbour, the
headlands outside are fringed by reefs apparently of coral; coloured
red.—_Mozambique_ (150° S.) The outer part of the island on which the
city is built, and the neighbouring islands, are fringed by
coral-reefs; coloured red. From the description given in Owen’s “Narr.”
(volume i., page 162), the shore from _Mozambique_ to _Delagoa Bay_
appears to be low and sandy; many of the shoals and islets off this
line of coast are of coral-formation; but from their small size and
lowness, it is not possible, from the charts, to know whether they are
truly fringed. Hence this portion of coast is left uncoloured, as are
likewise those parts more northward, of which no mention has been made
in the foregoing pages from the want of information.
PERSIAN GULF.—From the charts lately published on a large scale by the
East India Company, it appears that several parts, especially the
southern shores of this gulf, are fringed by coral-reefs; but as the
water is very shallow, and as there are numerous sandbanks, which are
difficult to distinguish on the chart from reefs, I have not coloured
the upper part red. Towards the mouth, however, where the water is
rather deeper, the islands of _Ormuz_ and _Larrack_, appear so
regularly fringed, that I have coloured them red. There are certainly
no atolls in the Persian Gulf. The shores of _Immaum_, and of the
promontory forming the southern headland of the Persian Gulf, seem to
be without reefs. The whole S.W. part (except one or two small patches)
of _Arabia Felix_, and the shores of _Socotra_ appear from the charts
and memoir of Captain Haines (“Geographical Journal,” 1839, page 125)
to be without any reefs. I believe there are no extensive coral-reefs
on any part of the coasts of _India_, except on the low promontory of
_Madura_ (as already mentioned) in front of Ceylon.
RED SEA.—My information is chiefly derived from the admirable charts
published by the East India Company in 1836, from personal
communication with Captain Moresby, one of the surveyors, and from the
excellent memoir, “Über die Natur der Corallen-Bänken des Rothen
Meeres,” by Ehrenberg. The plains immediately bordering the Red Sea
seem chiefly to consist of a sedimentary formation of the newer
tertiary period. The shore is, with the exception of a few parts,
fringed by coral-reefs. The water is generally profoundly deep close to
the shore; but this fact, which has attracted the attention of most
voyagers, seems to have no necessary connection with the presence of
reefs; for Captain Moresby particularly observed to me, that, in
latitude 24° 10′ on the eastern side, there is a piece of coast, with
very deep water close to it, without any reefs, but not differing in
other respects from the usual nature of the coast-line. The most
remarkable feature in the Red Sea is the chain of submerged banks,
reefs, and islands, lying some way from the shore, chiefly on the
eastern side; the space within being deep enough to admit a safe
navigation in small vessels. The banks are generally of an oval form,
and some miles in width; but some of them are very long in proportion
to their width. Captain Moresby informs me that any one, who had not
made actual plans of them, would be apt to think that they were much
more elongated than they really are. Many of them rise to the surface,
but the greater number lie from five to thirty fathoms beneath it, with
irregular soundings on them. They consist of sand and living coral;
coral on most of them, according to Captain Moresby, covering the
greater part of their surface. They extend parallel to the shore, and
they are not unfrequently connected in their middle parts by short
transverse banks with the mainland. The sea is generally profoundly
deep quite close to them, as it is near most parts of the coast of the
mainland; but this is not universally the case, for between latitude
15° and 17° the water deepens quite gradually from the banks, both on
the eastern and western shores, towards the middle of the sea. Islands
in many parts arise from these banks; they are low, flat-topped, and
consist of the same horizontally stratified formation with that forming
the plain-like margin of the mainland. Some of the smaller and lower
islands consist of mere sand. Captain Moresby informs me, that small
masses of rock, the remnants of islands, are left on many banks where
there is now no dry land. Ehrenberg also asserts that most of the
islets, even the lowest, have a flat abraded basis, composed of the
same tertiary formation: he believes that as soon as the surf wears
down the protuberant parts of a bank, just beneath the level of the
sea, the surface becomes protected from further abrasion by the growth
of coral, and he thus accounts for the existence of so many banks
standing on a level with the surface of this sea. It appears that most
of the islands are certainly decreasing in size.
The form of the banks and islands is most singular in the part just
referred to, namely, from latitude 15° to 17°, where the sea deepens
quite gradually: the _Dhalac_ group, on the western coast, is
surrounded by an intricate archipelago of islets and shoals; the main
island is very irregularly shaped, and it includes a bay seven miles
long, by four across, in which no bottom was found with 252 feet: there
is only one entrance into this bay, half a mile wide, and with an
island in front of it. The submerged banks on the eastern coast, within
the same latitudes, round _Farsan_ Island, are, likewise, penetrated by
many narrow creeks of deep water; one is twelve miles long, in the form
of a hatchet, in which, close to its broad upper end, soundings were
not struck with 360 feet, and its entrance is only half a mile wide: in
another creek of the same nature, but even with a more irregular
outline, there was no bottom with 480 feet. The island of Farsan,
itself, has as singular a form as any of its surrounding banks. The
bottom of the sea round the Dhalac and Farsan Islands consists chiefly
of sand and agglutinated fragments, but, in the deep and narrow creeks,
it consists of mud; the islands themselves consist of thin,
horizontally stratified, modern tertiary beds, containing but little
broken coral;[2] their shores are fringed by living coral-reefs.
[2] Rüppell, “Reise in Abyssinie,” Band. i., S. 247.
From the account given by Rüppell[3] of the manner in which Dhalac has
been rent by fissures, the opposite sides of which have been unequally
elevated (in one instance to the amount of fifty feet), it seems
probable that its irregular form, as well as probably that of Farsan,
may have been partly caused by unequal elevations; but, considering the
general form of the banks, and of the deep-water creeks, together with
the composition of the land, I think their configuration is more
probably due in great part to strong currents having drifted sediment
over an uneven bottom: it is almost certain that their form cannot be
attributed to the growth of coral. Whatever may have been the precise
origin of the Dhalac and Farsan Archipelagoes, the greater number of
the banks on the eastern side of the Red Sea seem to have originated
through nearly similar means. I judge of this from their similarity in
configuration (in proof of which I may instance a bank on the east
coast in latitude 22°; and although it is true that the northern banks
generally have a less complicated outline), and from their similarity
in composition, as may be observed in their upraised portions. The
depth within the banks northward of latitude 17°, is usually greater,
and their outer sides shelve more abruptly (circumstances which seem to
go together) than in the Dhalac and Farsan Archipelagoes; but this
might easily have been caused by a difference in the action of the
currents during their formation: moreover, the greater quantity of
living coral, which, according to Captain Moresby, exists on the
northern banks, would tend to give them steeper margins.
[3] Rüppell, “Reise in Abyssinie,” Band. i., S. 245.
From this account, brief and imperfect as it is, we can see that the
great chain of banks on the eastern coast, and on the western side in
the southern portion, differ greatly from true barrier-reefs wholly
formed by the growth of coral. It is indeed the direct conclusion of
Ehrenberg (“Uber die,” etc., pages 45 and 51), that they are connected
in their origin quite secondarily with the growth of coral; and he
remarks that the islands off the coast of Norway, if worn down level
with the sea, and merely coated with living coral, would present a
nearly similar appearance. I cannot, however, avoid suspecting, from
information given me by Dr. Malcolmson and Captain Moresby, that
Ehrenberg has rather under-rated the influence of corals, in some
places at least, on the formation of the tertiary deposits of the Red
Sea.
_The West Coast of the Red Sea between lat._ 19° _and_ 22°.—There are,
in this space, reefs, which, if I had known nothing of those in other
parts of the Red Sea, I should unhesitatingly have considered as
barrier-reefs; and, after deliberation, I have come to the same
conclusion. One of these reefs, in 20° 15′, is twenty miles long, less
than a mile in width (but expanding at the northern end into a disc),
slightly sinuous, and extending parallel to the mainland at the
distance of five miles from it, with very deep water within; in one
spot soundings were not obtained with 205 fathoms. Some leagues further
south, there is another linear reef, very narrow, ten miles long, with
other small portions of reef, north and south, almost connected with
it; and within this line of reefs (as well as outside) the water is
profoundly deep. There are also some small linear and sickle-formed
reefs, lying a little way out at sea. All these reefs are covered, as I
am informed by Captain Moresby, by living corals. Here, then, we have
all the characters of reefs of the barrier class; and in some outlying
reefs we have an approach to the structure of atolls. The source of my
doubts about the classification of these reefs, arises from having
observed in the Dhalac and Farsan groups the narrowness and
straightness of several spits of sand and rock: one of these spits in
the Dhalac group is nearly fifteen miles long, only two broad, and it
is bordered on each side with deep water; so that, if worn down by the
surf, and coated with living corals, it would form a reef nearly
similar to those within the space under consideration. There is, also,
in this space (latitude 21°) a peninsula, bordered by cliffs, with its
extremity worn down to the level of the sea, and its basis fringed with
reefs: in the line of prolongation of this peninsula, there lies the
island of _Macowa_ (formed, according to Captain Moresby, of the usual
tertiary deposit), and some smaller islands, large parts of which
likewise appear to have been worn down, and are now coated with living
corals. If the removal of the strata in these several cases had been
more complete, the reefs thus formed would have nearly resembled those
barrier-like ones now under discussion. Notwithstanding these facts, I
cannot persuade myself that the many very small, isolated, and
sickle-formed reefs and others, long, nearly straight, and very narrow,
with the water unfathomably deep close round them, could possibly have
been formed by corals merely coating banks of sediment, or the abraded
surfaces of irregularly shaped islands. I feel compelled to believe
that the foundations of these reefs have subsided, and that the corals,
during their upward growth, have given to these reefs their present
forms: I may remark that the subsidence of narrow and
irregularly-shaped peninsulas and islands, such as those existing on
the coasts of the Red Sea, would afford the requisite foundations for
the reefs in question.
_The West Coast from lat._ 22° _to_ 24°.—This part of the coast (north
of the space coloured blue on the map) is fronted by an irregularly
shelving bank, from about ten to thirty fathoms deep; numerous little
reefs, some of which have the most singular shapes, rise from this
bank. It may be observed, respecting one of them, in latitude 23° 10′,
that if the promontory in latitude 24° were worn down to the level of
the sea, and coated with corals, a very similar and grotesquely formed
reef would be produced. Many of the reefs on this part of the coast may
thus have originated; but there are some sickle, and almost
atoll-formed reefs lying in deep water off the promontory in latitude
24°, which lead me to suppose that all these reefs are more probably
allied to the barrier or atoll classes. I have not, however, ventured
to colour this portion of coast.—_On the west coast from lat._ 19° _to_
17° (south of space coloured blue on the map), there are many low
islets of very small dimensions, not much elongated, and rising out of
great depths at a distance from the coast; these cannot be classed
either with atolls, or barrier- or fringing-reefs. I may here remark
that the outlying reefs on the west coast, between latitude 19° and
24°, are the only ones in the Red Sea, which approach in structure to
the true atolls of the Indian and Pacific Oceans, but they present only
imperfect miniature likenesses of them.
_Eastern Coast_.—I have felt the greatest doubt about colouring any
portion of this coast, north of the fringing-reefs round the Farsan
Islands in 16° 10′. There are many small outlying coral-reefs along the
whole line of coast; but as the greater number rise from banks not very
deeply submerged (the formation of which has been shown to be only
secondarily connected with the growth of coral), their origin may be
due simply to the growth of knolls of corals, from an irregular
foundation situated within a limited depth. But between latitude 18°
and 20°, there are so many linear, elliptic, and extremely small reefs,
rising abruptly out of profound depths, that the same reasons, which
led me to colour blue a portion of the west coast, have induced me to
do the same in this part. There exist some small outlying reefs rising
from deep water, north of latitude 20° (the northern limit coloured
blue), on the east coast; but as they are not very numerous and
scarcely any of them linear, I have thought it right to leave them
uncoloured.
In the _southern parts_ of the Red Sea, considerable spaces of the
mainland, and of some of the Dhalac islands, are skirted by reefs,
which, as I am informed by Captain Moresby, are of living coral, and
have all the characters of the fringing class. As in these latitudes,
there are no outlying linear or sickle-formed reefs, rising out of
unfathomable depths, I have coloured these parts of the coast red. On
similar grounds, I have coloured red the _northern parts of the western
coast_ (north of latitude 24° 30′), and likewise the shores of the
chief part of the _Gulf of Suez_. In the _Gulf of Acaba_, as I am
informed by Captain Moresby there are no coral-reefs, and the water is
profoundly deep.
WEST INDIES.—My information regarding the reefs of this area, is
derived from various sources, and from an examination of numerous
charts; especially of those lately executed during the survey under
Captain Owen, R.N. I lay under particular obligation to Captain Bird
Allen, R.N., one of the members of the late survey, for many personal
communications on this subject. As in the case of the Red Sea, it is
necessary to make some preliminary remarks on the submerged banks of
the West Indies, which are in some degree connected with coral-reefs,
and cause considerable doubts in their classification. That large
accumulations of sediment are in progress on the West Indian shores,
will be evident to any one who examines the charts of that sea,
especially of the portion north of a line joining Yucutan and Florida.
The area of deposition seems less intimately connected with the
debouchement of the great rivers, than with the course of the
sea-currents; as is evident from the vast extension of the banks from
the promontories of Yucutan and Mosquito.
Besides the coast-banks, there are many of various dimensions which
stand quite isolated; these closely resemble each other, they lie from
two or three to twenty or thirty fathoms under water, and are composed
of sand, sometimes firmly agglutinated, with little or no coral; their
surfaces are smooth and nearly level, shelving only to the amount of a
few fathoms, very gradually all round towards their edges, where they
plunge abruptly into the unfathomable sea. This steep inclination of
their sides, which is likewise characteristic of the coast-banks, is
very remarkable: I may give as an instance, the Misteriosa Bank, on the
edges of which the soundings change in 250 fathoms horizontal distance,
from 11 to 210 fathoms; off the northern point of the bank of Old
Providence, in 200 fathoms horizontal distance, the change is from 19
to 152 fathoms; off the Great Bahama Bank, in 160 fathoms horizontal
distance, the inclination is in many places from 10 fathoms to no
bottom with 190 fathoms. On coasts in all parts of the world, where
sediment is accumulating, something of this kind may be observed; the
banks shelve very gently far out to sea, and then terminate abruptly.
The form and composition of the banks standing in the middle parts of
the W. Indian Sea, clearly show that their origin must be chiefly
attributed to the accumulation of sediment; and the only obvious
explanation of their isolated position is the presence of a nucleus,
round which the currents have collected fine drift matter. Any one who
will compare the character of the bank surrounding the hilly island of
Old Providence, with those banks in its neighbourhood which stand
isolated, will scarcely doubt that they surround submerged mountains.
We are led to the same conclusion by examining the bank called Thunder
Knoll, which is separated from the Great Mosquito Bank by a channel
only seven miles wide, and 145 fathoms deep. There cannot be any doubt
that the Mosquito Bank has been formed by the accumulation of sediment
round the promontory of the same name; and Thunder Knoll resembles the
Mosquito Bank, in the state of its surface submerged twenty fathoms, in
the inclinations of its sides, in composition, and in every other
respect. I may observe, although the remark is here irrelevant, that
geologists should be cautious in concluding that all the outlyers of
any formation have once been connected together, for we here see that
deposits, doubtless of exactly the same nature, may be deposited with
large valley-like spaces between them.
Linear strips of coral-reefs and small knolls project from many of the
isolated, as well as coast-banks; sometimes they occur quite
irregularly placed, as on the Mosquito Bank, but more generally they
form crescents on the windward side, situated some little distance
within the outer edge of the banks:—thus on the Serranilla Bank they
form an interrupted chain which ranges between two and three miles
within the windward margin: generally they occur, as on Roncador,
Courtown, and Anegada Banks, nearer the line of deep water. Their
occurrence on the windward side is conformable to the general rule, of
the efficient kinds of corals flourishing best where most exposed; but
their position some way within the line of deep water I cannot explain,
without it be, that a depth somewhat less than that close to the outer
margin of the banks, is most favourable to their growth. Where the
corals have formed a nearly continuous rim, close to the windward edge
of a bank some fathoms submerged, the reef closely resembles an atoll;
but if the bank surrounds an island (as in the case of Old Providence),
the reef resembles an encircling barrier-reef. I should undoubtedly
have classed some of these fringed banks as imperfect atolls, or
barrier-reefs, if the sedimentary nature of their foundations had not
been evident from the presence of other neighbouring banks, of similar
forms and of similar composition, but without the crescent-like
marginal reef: in the third chapter, I observed that probably some
atoll-like reefs did exist, which had originated in the manner here
supposed.
Proofs of elevation within recent tertiary periods abound, as referred
to in the sixth chapter, over nearly the whole area of the West Indies.
Hence it is easy to understand the origin of the low land on the
coasts, where sediment is now accumulating; for instance on the
northern part of Yucutan, and on the N.E. part of Mosquito, where the
land is low, and where extensive banks appear to be in progressive
formation. Hence, also, the origin of the Great Bahama Banks, which are
bordered on their western and southern edges by very narrow, long,
singularly shaped islands, formed of sand, shells, and coral-rock, and
some of them about a hundred feet in height, is easily explained by the
elevation of banks fringed on their windward (western and southern)
sides by coral-reefs. On this view, however, we must suppose either
that the chief part of the surfaces of the great Bahama sandbanks were
all originally deeply submerged, and were brought up to their present
level by the same elevatory action, which formed the linear islands; or
that during the elevation of the banks, the superficial currents and
swell of the waves continued wearing them down and keeping them at a
nearly uniform level: the level is not quite uniform; for, in
proceeding from the N.W. end of the Bahama group towards the S.E. end,
the depth of the banks increases, and the area of land decreases, in a
very gradual and remarkable manner. The latter view, namely, that these
banks have been worn down by the currents and swell during their
elevation, seems to me the most probable one. It is, also, I believe,
applicable to many banks, situated in widely distant parts of the West
Indian Sea, which are wholly submerged; for, on any other view, we must
suppose, that the elevatory forces have acted with astonishing
uniformity.
The shore of the Gulf of Mexico, for the space of many hundred miles,
is formed by a chain of lagoons, from one to twenty miles in breadth
(“Columbian Navigator,” page 178, etc.), containing either fresh or
salt water, and separated from the sea by linear strips of sand. Great
spaces of the shores of Southern Brazil.[4] and of the United States
from Long Island (as observed by Professor Rogers) to Florida have the
same character. Professor Rogers, in his “Report to the British
Association” (volume iii., page 13), speculates on the origin of these
low, sandy, linear islets; he states that the layers of which they are
composed are too homogeneous, and contain too large a proportion of
shells, to permit the common supposition of their formation being
simply due to matter thrown up, where it now lies, by the surf: he
considers these islands as upheaved bars or shoals, which were
deposited in lines where opposed currents met. It is evident that these
islands and spits of sand parallel to the coast, and separated from it
by shallow lagoons, have no necessary connection with coral-formations.
But in Southern Florida, from the accounts I have received from persons
who have resided there, the upraised islands seem to be formed of
strata, containing a good deal of coral, and they are extensively
fringed by living reefs; the channels within these islands are in some
places between two and three miles wide, and five or six fathoms deep,
though generally[5] they are less in depth than width. After having
seen how frequently banks of sediment in the West Indian Sea are
fringed by reefs, we can readily conceive that bars of sediment might
be greatly aided in their formation along a line of coast, by the
growth of corals; and such bars would, in that case, have a deceptive
resemblance with true barrier-reefs.
[4] In the “London and Edinburgh Philosophical Journal,” 1841, page
257, I have described a singular bar of sandstone lying parallel to
the coast off Pernambuco in Brazil, which probably is an analogous
formation.
[5] In the ordinary sea-charts, no lagoons appear on the coast of
Florida, north of 26°; but Major Whiting (“Silliman’s Journal,” volume
xxxv., page 54) says that many are formed by sand thrown up along the
whole line of coast from St. Augustine’s to Jupiter Inlet.
Having now endeavoured to remove some sources of doubt in classifying
the reefs of the West Indies, I will give my authorities for colouring
such portions of the coast as I have thought myself warranted in doing.
Captain Bird Allen informs me, that most of the islands on the _Bahama
Banks_ are fringed, especially on their windward sides, with living
reefs; and hence I have coloured those, which are thus represented in
Captain Owen’s late chart, red. The same officer informs me, that the
islands along the southern part of _Florida_ are similarly fringed;
coloured red. CUBA: Proceeding along the northern coast, at the
distance of forty miles from the extreme S.E. point, the shores are
fringed by reefs, which extend westward for a space of 160 miles, with
only a few breaks. Parts of these reefs are represented in the plans of
the harbours on this coast by Captain Owen; and an excellent
description is given of them by Mr. Taylor (Loudon’s “Mag. of Nat.
Hist.” volume ix., page 449); he states that they enclosed a space
called the “_baxo_,” from half to three-quarters of a mile in width,
with a sandy bottom, and a little coral. In most parts people can wade,
at low water, to the reef; but in some parts the depth is between two
and three fathoms. Close outside the reef, the depth is between six and
seven fathoms; these well-characterised fringing-reefs are coloured
red. Westward of longitude 77° 30′, on the northern side of Cuba, a
great bank commences, which extends along the coast for nearly four
degrees of longitude. In the place of its commencement, in its
structure, and in the “_cays_,” or low islands on its edge, there is a
marked correspondence (as observed by Humboldt, “Pers. Narr.” volume
vii., page 88) between it and the Great Bahama and Sal Banks, which lie
directly in front. Hence one is led to attribute the same origin to
both these sets of banks; namely, the accumulation of sediment,
conjoined with an elevatory movement, and the growth of coral on their
outward edges; those parts which appear fringed by living reefs are
coloured red. Westward of these banks, there is a portion of coast
apparently without reefs, except in the harbours, the shores of which
seem in the published plans to be fringed. The _Colorado Shoals_ (see
Captain Owen’s charts), and the low land at the western end of Cuba,
correspond as closely in relative position and structure to the banks
at the extreme point of Florida, as the banks above described on the
north side of Cuba, do to the Bahamas, the depth within the islets and
reefs on the outer edge of the _Colorados_, is generally between two
and three fathoms, increasing to twelve fathoms in the southern part,
where the bank becomes nearly open, without islets or coral-reefs; the
portions which are fringed are coloured red. The southern shore of Cuba
is deeply concave, and the included space is filled up with mud and
sandbanks, low islands and coral-reefs. Between the mountainous _Isle
of Pines_ and the southern shore of Cuba, the general depth is only
between two and three fathoms; and in this part small islands, formed
of fragmentary rock and broken madrepores (Humboldt, “Pers. Narr.”
volume vii. pages 51, 86 to 90, 291, 309, 320), rise abruptly, and just
reach the surface of the sea. From some expressions used in the
“Columbian Navigator” (volume i., part ii., page 94), it appears that
considerable spaces along the outer coast of Southern Cuba are bounded
by cliffs of coral-rock, formed probably by the upheaval of coral-reefs
and sandbanks. The charts represent the southern part of the Isle of
Pines as fringed by reefs, which the “Columb. Navig.” says extend some
way from the coast, but have only from nine to twelve feet water on
them; these are coloured red.—I have not been able to procure any
detailed description of the large groups of banks and “cays” further
eastward on the southern side of Cuba; within them there is a large
expanse, with a muddy bottom, from eight to twelve fathoms deep;
although some parts of this line of coast are represented in the
general charts of the West Indies, as fringed, I have not thought it
prudent to colour them. The remaining portion of the south coast of
Cuba appears to be without coral-reefs.
YUCUTAN.—The N.E. part of the promontory appears in Captain Owen’s
charts to be fringed; coloured red. The eastern coast, from 20° to 18°
is fringed. South of latitude 18°, there commences the most remarkable
reef in the West Indies: it is about one hundred and thirty miles in
length, ranging in a N. and S. line, at an average distance of fifteen
miles from the coast. The islets on it are all low, as I have been
informed by Captain B. Allen; the water deepens suddenly on the outside
of the reef, but not more abruptly than off many of the sedimentary
banks: within its southern extremity (off _Honduras_) the depth is
twenty-five fathoms; but in the more northern parts, the depth soon
increases to ten fathoms, and within the northernmost part, for a space
of twenty miles, the depth is only from one to two fathoms. In most of
these respects we have the characteristics of a barrier-reef;
nevertheless, from observing, first, that the channel within the reef
is a continuation of a great irregular bay, which penetrates the
mainland to the depth of fifty miles; and secondly, that considerable
spaces of this barrier-like reef are described in the charts (for
instance, in latitude 16° 45′ and 16° 12′) as formed of pure sand; and
thirdly, from knowing that sediment is accumulating in many parts of
the West Indies in banks parallel to the shore; I have not ventured to
colour this reef as a barrier, without further evidence that it has
really been formed by the growth of corals, and that it is not merely
in parts a spit of sand, and in other parts a worn down promontory,
partially coated and fringed by reefs; I lean, however, to the
probability of its being a barrier-reef, produced by subsidence. To add
to my doubts, immediately on the outside of this barrier-like reef,
_Turneffe_, _Lighthouse_, and _Glover_ reefs are situated, and these
reefs have so completely the form of atolls, that if they had occurred
in the Pacific, I should not have hesitated about colouring them blue.
_turneffe Reef_ seems almost entirely filled up with low mud islets;
and the depth within the other two reefs is only from one to three
fathoms. From this circumstance and from their similarity in form,
structure, and relative position, both to the bank called _Northern
Triangles_, on which there is an islet between seventy and eighty feet,
and to _Cozumel_ Island, the level surface of which is likewise between
seventy and eighty feet in height, I consider it more probable that the
three foregoing banks are the worn down bases of upheaved shoals,
fringed with corals, than that they are true atolls, wholly produced by
the growth of coral during subsidence; left uncoloured.
In front of the eastern _Mosquito_ coast, there are between latitude
12° and 16° some extensive banks (already mentioned, page 148), with
high islands rising from their centres; and there are other banks
wholly submerged, both of which kinds of banks are bordered, near their
windward margins, by crescent-shaped coral-reefs. But it can hardly be
doubted, as was observed in the preliminary remarks, that these banks
owe their origin, like the great bank extending from the Mosquito
promontory, almost entirely to the accumulation of sediment, and not to
the growth of corals; hence I have not coloured them.
_Cayman Island_: this island appears in the charts to be fringed; and
Captain B. Allen informs me that the reefs extend about a mile from the
shore, and have only from five to twelve feet water within them;
coloured red.—_Jamaica_: judging from the charts, about fifteen miles
of the S.E. extremity, and about twice that length on the S.W.
extremity, and some portions on the S. side near Kingston and Port
Royal, are regularly fringed, and therefore are coloured red. From the
plans of some harbours on the N. side of Jamaica, parts of the coast
appear to be fringed; but as these are not represented in the charts of
the whole island, I have not coloured them.—_St. Domingo_: I have not
been able to obtain sufficient information, either from plans of the
harbours, or from general charts, to enable me to colour any part of
the coast, except sixty miles from Port de Plata westward, which seems
very regularly fringed; many other parts, however, of the coast are
probably fringed, especially towards the eastern end of the
island.—_Puerto Rico_: considerable portions of the southern, western,
and eastern coasts, and some parts of the northern coast, appear in the
charts to be fringed; coloured red.—Some miles in length of the
southern side of the Island of _St, Thomas_ is fringed; most of the
_Virgin Gorda_ Islands, as I am informed by Mr. Schomburgk, are
fringed; the shores of _Anegada_, as well as the bank on which it
stands, are likewise fringed; these islands have been coloured red. The
greater part of the southern side of _Santa Cruz_ appears in the Danish
survey to be fringed (see also Prof. Hovey’s account of this island, in
“Silliman’s Journal,” volume xxxv., page 74); the reefs extend along
the shore for a considerable space, and project rather more than a
mile; the depth within the reef is three fathoms; coloured red.—The
_Antilles_, as remarked by Von Buch (“Descrip. Iles Canaries,” page
494), may be divided into two linear groups, the western row being
volcanic, and the eastern of modern calcareous origin; my information
is very defective on the whole group. Of the eastern islands, _Barbuda_
and the western coasts of _Antigua_ and _Mariagalante_ appear to be
fringed: this is also the case with _Barbadoes_, as I have been
informed by a resident; these islands are coloured red. On the shores
of the Western Antilles, of volcanic origin, very few coral-reefs
appear to exist. The island of _Martinique_, of which there are
beautifully executed French charts, on a very large scale, alone
presents any appearance worthy of special notice. The south-western,
southern, and eastern coasts, together forming about half the
circumference of the island, are skirted by very irregular banks,
projecting generally rather less than a mile from the shore, and lying
from two to five fathoms submerged. In front of almost every valley,
they are breached by narrow, crooked, steep-sided passages. The French
engineers ascertained by boring, that these submerged banks consisted
of madreporitic rocks, which were covered in many parts by thin layers
of mud or sand. From this fact, and especially from the structure of
the narrow breaches, I think there can be little doubt that these banks
once formed living reefs, which fringed the shores of the island, and
like other reefs probably reached the surface. From some of these
submerged banks reefs of living coral rise abruptly, either in small
detached patches, or in lines parallel to, but some way within the
outer edges of the banks on which they are based. Besides the above
banks which skirt the shores of the island, there is on the eastern
side a range of linear banks, similarly constituted, twenty miles in
length, extending parallel to the coast line, and separated from it by
a space between two and four miles in width, and from five to fifteen
fathoms in depth. From this range of detached banks, some linear reefs
of living coral likewise rise abruptly; and if they had been of greater
length (for they do not front more than a sixth part of the
circumference of the island), they would necessarily from their
position have been coloured as barrier-reefs; as the case stands they
are left uncoloured. I suspect that after a small amount of subsidence,
the corals were killed by sand and mud being deposited on them, and the
reefs being thus prevented from growing upwards, the banks of
madreporitic rock were left in their present submerged condition.
THE BERMUDA ISLANDS have been carefully described by Lieutenant Nelson,
in an excellent Memoir in the “Geological Transactions” (volume v.,
part i., page 103). In the form of the bank or reef, on one side of
which the islands stand, there is a close general resemblance to an
atoll; but in the following respects there is a considerable
difference,—first, in the margin of the reef not forming (as I have
been informed by Mr. Chaffers, R.N.) a flat, solid surface, laid bare
at low water, and regularly bounding the internal space of shallow
water or lagoon; secondly, in the border of gradually shoaling water,
nearly a mile and a half in width, which surrounds the entire outside
of the reef (as is laid down in Captain Hurd’s chart); and thirdly, in
the size, height, and extraordinary form of the islands, which present
little resemblance to the long, narrow, simple islets, seldom exceeding
half a mile in breadth, which surmount the annular reefs of almost all
the atolls in the Indian and Pacific Oceans. Moreover, there are
evident proofs (Nelson, Ibid., page 118), that islands similar to the
existing ones, formerly extended over other parts of the reef. It
would, I believe, be difficult to find a true atoll with land exceeding
thirty feet in height; whereas, Mr. Nelson estimates the highest point
of the Bermuda Islands to be 260 feet; if, however, Mr. Nelson’s view,
that the whole of the land consists of sand drifted by the winds, and
agglutinated together, were proved correct, this difference would be
immaterial; but, from his own account (page 118), there occur in one
place, five or six layers of red earth, interstratified with the
ordinary calcareous rock, and including stones too heavy for the wind
to have moved, without having at the same time utterly dispersed every
grain of the accompanying drifted matter. Mr. Nelson attributes the
origin of these several layers, with their embedded stones, to as many
violent catastrophes; but further investigation in such cases has
generally succeeded in explaining phenomena of this kind by ordinary
and simpler means. Finally, I may remark, that these islands have a
considerable resemblance in shape to Barbuda in the West Indies, and to
Pemba on the eastern coast of Africa, which latter island is about two
hundred feet in height, and consists of coral-rock. I believe that the
Bermuda Islands, from being fringed by living reefs, ought to have been
coloured red; but I have left them uncoloured, on account of their
general resemblance in external form to a lagoon-island or atoll.
INDEX.
The names not in capitals are all names of places, and refer
exclusively to the Appendix: in well-defined archipelagoes, or groups
of islands, the name of each separate island is not given.
ABROLHOS, Brazil, coated by corals.
Abrolhos (Australia).
ABSENCE of coral-reefs from certain coasts.
Acaba, gulf of.
Admiralty group.
AFRICA, east coast, fringing-reef of.
Madreporitic rock of.
Africa, east coast.
AGE of individual corals.
Aiou.
Aitutaki.
Aldabra.
Alert reef.
Alexander, Grand Duke, island.
ALLAN, Dr., on Holuthuriæ feeding on corals. On quick growth of corals
at Madagascar. On reefs affected by currents.
Alloufatou.
Alphonse.
Amargoura. (Amargura.)
Amboina.
America, west coast.
Amirantes.
Anachorites.
Anambas.
ANAMOUKA, description of.
Anamouka.
Anadaman islands.
Antilles.
Appoo reef.
Arabia Felix.
AREAS, great extent of, interspersed with low islands. Of subsidence
and of elevation. Of subsidence appear to be elongated. Of subsidence
alternating with areas of elevation.
Arru group.
Arzobispo.
ASCIDIA, depth at which found.
Assomption.
Astova.
Atlantic islands.
ATOLLS, breaches in their reefs. Dimensions of. Dimensions of groups
of. Not based on craters or on banks of sediment, or of rock. Of
irregular forms. Steepness of their flanks. Width of their reef and
islets. Their lowness. Lagoons. General range. With part of their reef
submerged, and theory of.
Augustine, St.
AURORA island, an upraised atoll.
Aurora.
AUSTRAL islands, recently elevated.
Austral islands.
Australia, N.W. coast.
AUSTRALIAN barrier-reef.
Australian barrier.
Babuyan group.
Bahama banks.
Balahac.
Bally.
Baring.
BARRIER-REEF of Australia. Of New Caledonia.
BARRIER-REEFS, breaches through. Not based on worn down margin of rock.
On banks of sediment. On submarine craters. Steepness of their flanks.
Their probable vertical thickness. Theory of their formation.
Bampton shoal.
Banks islands.
Banks in the West Indies.
Bashee islands.
Bass island.
Batoa.
Beaupre reef.
BEECHEY, Captain, obligations of the author to. On submerged reefs.
Account of Matilda island.
BELCHER, Captain, on boring through coral-reef.
Belize reef, off.
Bellinghausen.
Bermuda islands.
Beveridge reef.
Bligh.
BOLABOLA, view of.
Bombay shoal.
Bonin Bay.
Bonin group.
BORINGS through coral-reefs.
BORNEO, W. coast, recently elevated.
Borneo, E. coast. S.W. and W. coast N. coast. Western bank.
Boscawen.
Boston.
Bouka.
Bourbon.
Bourou.
Bouton.
BRAZIL, fringing-reefs on coast of.
BREACHES through barrier-reefs.
Brook.
Bunker.
Bunoa.
BYRON.
Cagayanes.
Candelaria.
Cargados Carajos.
Caroline archipelago.
Caroline island.
Carteret shoal.
CARYOPHYLLIA, depth at which it lives.
Cavilli.
Cayman island.
Celebes.
Ceram.
CEYLON, recently elevated.
Ceylon.
CHAGOS Great Bank, description and theory of.
CHAGOS group.
Chagos group.
CHAMA-SHELLS embedded in coral-rock.
CHAMISSO, on corals preferring the surf.
CHANGES in the state of Keeling atoll. Of atolls.
CHANNELS leading into the lagoons of atolls. Into the Maldiva atolls.
Through barrier-reefs.
Chase.
China sea.
CHRISTMAS atoll.
Christmas atoll.
Christmas island (Indian Ocean).
Clarence.
Clipperton rock.
COCOS, or Keeling atoll.
Cocos (or Keeling).
Cocos island (Pacific).
COCHIN China, encroachments of the sea on the coast.
Cochin China.
Coetivi.
Comoro group.
COMPOSITION of coral-formations.
CONGLOMERATE coral-rock on Keeling atoll. On other atolls. Coral-rock.
COOK islands, recently elevated.
Cook islands.
CORAL-BLOCKS bored by vermiform animals.
CORAL-REEFS, their distribution and absence from certain areas.
Destroyed by loose sediment.
CORAL-ROCK at Keeling atoll. Mauritius. Organic remains of.
CORALS dead but upright in Keeling lagoon. Depths at which they live.
Off Keeling atoll. Killed by a short exposure. Living in the lagoon of
Keeling atoll. Quick growth of, in Keeling lagoon. Merely coating the
bottom of the sea. Standing exposed in the Low archipelago.
CORALLIAN sea.
Corallian sea.
Cornwallis.
Cosmoledo.
COUTHOUY, Mr., alleged proofs of recent elevation of the Low
archipelago. On coral-rock at Mangaia and Aurora islands. On external
ledges round coral-islands. Remarks confirmatory of the author’s
theory.
CRESCENT-FORMED reefs.
Cuba.
CUMING, Mr., on the recent elevation of the Philippines.
Dangerous, or Low archipelago.
Danger islands.
DEPTHS at which reef-building corals live. At Mauritius, the Red Sea,
and in the Maldiva archipelago. At which other corals and corallines
can live.
Dhalac group.
DIEGO GARCIA, slow growth of reef.
DIMENSIONS of the larger groups of atolls.
DISSEVERMENT of the Maldiva atolls, and theory of.
DISTRIBUTION of coral-reefs.
Domingo, St.
DORY, Port, recently elevated.
Dory, Port.
Duff islands.
Durour.
Eap.
EARTHQUAKES at Keeling atoll. In groups of atolls. In Navigator
archipelago.
EAST INDIAN ARCHIPELAGO, recently elevated.
Easter.
Echequier.
EHRENBERG, on the banks of the Red Sea. On depths at which corals live
in the Red Sea. On corals preferring the surf. On the antiquity of
certain corals.
Eimeo.
ELEVATED reef of Mauritius.
ELEVATIONS, recent proofs of. Immense areas of.
Elivi.
ELIZABETH island. Recently elevated.
Elizabeth island.
Ellice group.
ENCIRCLED ISLANDS, their height. Geological composition.
EOUA, description of.
Eoua.
ERUPTED MATTER probably not associated with thick masses of coral-rock.
FAIS, recently elevated.
Fais.
Fanning.
Farallon de Medinilla.
Farson group.
Fataka.
FIJI archipelago.
FISH, feeding on corals. Killed in Keeling lagoon by heavy rain.
FISSURES across coral-islands.
FITZROY, Captain, on a submerged shed at Keeling atoll. On an
inundation in the Low archipelago.
Flint.
Flores.
Florida.
Folger.
Formosa.
FORSTER, theory of coral-formations.
Frederick reef.
Freewill.
FRIENDLY group recently elevated.
Friendly archipelago.
FRINGING-REEFS, absent where coast precipitous. Breached in front of
streams. Described by MM. Quoy and Gaimard. Not closely attached to
shelving coasts. Of east coast of Africa. Of Cuba. Of Mauritius. On
worn down banks of rock. On banks of sediment. Their appearance when
elevated. Their growth influenced by currents. By shallowness of sea.
Galapagos archipelago.
Galega.
GAMBIER islands, section of.
Gambier islands.
Gardner.
Gaspar rico.
GEOLOGICAL COMPOSITION of coral-formations.
Gilbert archipelago.
Gilolo.
Glorioso.
GLOUCESTER Island.
Glover reef.
Gomez.
Gouap.
Goulou.
Grampus.
Gran Cocal.
GREAT CHAGOS BANK, description and theory of.
GREY, Captain, on sandbars.
GROUPING of the different classes of reefs.
Guedes.
HALL, Captain B., on Loo Choo.
HARVEY islands, recently elevated.
HEIGHT of encircled islands.
Hermites.
Hervey or Cook islands.
Hogoleu.
HOLOTHURIAE (Holuthuriæ) feeding on coral.
HOUDEN island, height of.
Honduras, reef off.
Horn.
Houtman Abrolhos.
HUAHEINE; alleged proofs of its recent elevation.
Huaheine.
Humphrey.
Hunter.
HURRICANES, effects of, on coral-islands.
Immaum.
Independence.
INDIA, west coast, recently elevated.
India.
IRREGULAR REEFS in shallow seas.
ISLETS of coral-rock, their formation. Their destruction in the Maldiva
atolls.
Jamaica.
Jarvis.
JAVA, recently elevated.
Java.
Johnston island.
Juan de Nova.
Juan de Nova (Madagascar).
Kalatoa.
KAMTSCHATKA, proofs of its recent elevation.
Karkalang.
KEELING atoll, section of reef.
Keeling, south atoll. North atoll.
Keffing.
Kemin.
Kennedy.
Keppel.
Kumi.
Laccadive group.
LADRONES, or Marianas, recently elevated.
Ladrones archipelago.
LAGOON of Keeling atoll.
LAGOONS bordered by inclined ledges and walls, and theory of their
formation. Of small atolls filled up with sediment.
LAGOON-CHANNELS within barrier-reefs.
LAGOON-REEFS, all submerged in some atolls, and rising to the surface
in others.
Lancaster reef.
Latte.
Lauglan islands.
LEDGES round certain lagoons.
Lette.
Lighthouse reef.
LLOYD, Mr., on corals refixing themselves.
LOO CHOO, recently elevated.
Loo Choo.
Louisiade.
LOW ARCHIPELAGO, alleged proofs of its recent elevation.
Low archipelago.
LOWNESS of coral-islands.
Loyalty group.
Lucepara.
LUTKE, Admiral, on fissures across coral-islands.
LUZON, recently elevated.
Luzon.
LYELL, Mr., on channels into the lagoons of atolls. On the lowness of
their leeward sides. On the antiquity of certain corals. On the
apparent continuity of distinct coral-islands. On the recently elevated
beds of the Red Sea. On the outline of the areas of subsidence.
Macassar strait.
Macclesfield bank.
MADAGASCAR, quick growth of corals at. Madreporitic rock of.
Madagascar.
Madjiko-sima.
Madura (Java).
Madura (India).
MAHLOS MAHDOO, theory of formation.
MALACCA, recently elevated.
Malacca.
MALCOLMSON, Dr., on recent elevation of W. coast of India. On recent
elevation of Camaran island.
Malden.
MALDIVA atolls, and theory of their formation. Steepness of their
flanks. Growth of coral at.
Maldiva archipelago.
MANGAIA island. Recently elevated.
Mangaia.
Mangs.
MARIANAS, recently elevated.
Mariana archipelago.
Mariere.
Marquesas archipelago.
Marshall archipelago.
Marshall island.
Martinique.
Martires.
MARY’S ST., in Madagascar, harbour made in reefs.
Mary island.
Matia, or Aurora.
MATILDA atoll.
MAURITIUS, fringing-reefs of. Depths at which corals live there.
Recently elevated.
Mauritius.
MAURUA, section of.
Maurua.
MENCHIKOFF atoll.
Mendana archipelago.
Mendana isles.
Mexico, gulf of.
MILLEPORA COMPLANATA at Keeling atoll.
Mindoro.
Mohilla. (Mohila.)
MOLUCCA islands, recently elevated.
Mopeha.
MORESBY, Captain, on boring through coral-reefs.
Morty.
Mosquito coast.
MUSQUILLO atoll.
Mysol.
NAMOURREK group.
Natunas.
NAVIGATOR archipelago, elevation of.
Navigator archipelago.
Nederlandisch.
NELSON, Lieutenant, on the consolidation of coral-rocks under water.
Theory of coral-formations. On the Bermuda islands.
New Britain.
NEW CALEDONIA, steepness of its reefs. Barrier-reef of.
New Caledonia.
New Guinea (E. end).
New Guinea (W. end).
New Hanover.
NEW HEBRIDES, recently elevated.
New Hebrides.
NEW IRELAND, recently elevated.
New Ireland.
New Nantucket.
Nicobar islands.
Niouha.
NULLIPORAE at Keeling atoll. On the reefs of atolls. On barrier-reefs.
Their wide distribution and abundance.
OBJECTIONS to the theory of subsidence.
Ocean islands.
Ono.
Onouafu. (Onouafou.)
Ormuz.
Oscar group.
OSCILLATIONS of level.
Ouallan, or Ualan. (Oualan.)
OULUTHY atoll.
Outong Java.
Palawan, S.W. coast. N.W. coast. Western bank.
Palmerston.
Palmyra.
Paracells.
Paraquas.
Patchow.
Pelew islands.
PEMBA island, singular form of.
Pemba.
Penrhyn.
Peregrino.
PERNAMBUCO, bar of sandstone at.
PERSIAN gulf, recently elevated.
Persian gulf.
PESCADO.
Pescadores.
Peyster group.
Philip.
PHILIPPINE archipelago, recently elevated.
Philippine archipelago.
Phœnix.
Piguiram.
Pitcairn.
PITT’S bank.
Pitt island.
Platte.
Pleasant.
PORITES, chief coral on margin of Keeling atoll.
Postillions.
POUYNIPETE. Its probable subsidence.
Pouynipète.
Pratas shoal.
Proby.
Providence.
Puerto Rico.
Pulo Anna.
PUMICE floated to coral-islands.
Pylstaart.
PYRARD DE LAVAL, astonishment at the atolls in the Indian Ocean.
QUOY AND GAIMARD, depths at which corals live. Description of reefs
applicable only to fringing-reefs.
RANGE of atolls.
Rapa.
Rearson.
RED SEA, banks of rock coated by reefs. Proofs of its recent elevation.
Supposed subsidence of.
Red Sea.
REEFS, irregular in shallow seas. Rising to the surface in some lagoons
and all submerged in others. Their distribution. Their absence from
some coasts.
Revilla-gigedo.
RING-FORMED REEFS of the Maldiva atolls, and theory of.
Rodriguez.
Rosario.
Rose island.
Rotches.
Rotoumah.
Roug.
Rowley shoals.
RUPPELL, Dr., on the recent deposits of Red Sea.
Sable, ile de.
Sahia de Malha.
St. Pierre.
Sala.
Salomon archipelago. (Solomon.)
SAMOA, or Navigator archipelago, elevation of.
Samoa archipelago.
SAND-BARS parallel to coasts.
Sandal-wood.
SANDWICH archipelago, recently elevated.
Sandwich archipelago.
Sanserot.
Santa-Cruz group.
SAVAGE island, recently elevated.
Savage.
Savu.
Saya, or Sahia de Malha.
Scarborough shoal.
SCARUS feeding on corals.
Schouten.
Scilly.
SCORIAE floated to coral-islands.
Scott’s reef.
SECTIONS of islands encircled by barrier-reefs. Of Bolabola.
SEDIMENT in Keeling lagoon. In other atolls. Injurious to corals.
Transported from coral-islands far seaward.
Seniavine.
Serangani.
Seychelles.
SHIP-BOTTOM quickly coated with coral.
SMYTH island.
SOCIETY archipelago, stationary condition of. Alleged proofs of recent
elevation.
Society archipelago.
Socotra.
Solor.
SOOLOO islands, recently elevated.
Sooloo islands.
Souvaroff.
Spanish.
SPONGE, depths at which found.
Starbuck. (Slarbuck.)
STONES transported in roots of trees.
STORMS, effects of, on coral-islands.
STUTCHBURY, Mr., on the growth of an Agaricia. On upraised corals in
Society archipelago.
SUBSIDENCE of Keeling atoll. Extreme slowness of. Areas of, apparently
elongated. Areas of immense. Great amount of.
Suez, gulf of.
Sulphur islands.
SUMATRA, recently elevated.
Sumatra.
Sumbawa.
SURF favourable to the growth of massive corals.
Swallow shoal.
Sydney island.
TAHITI, alleged proofs of its recent elevation.
Tahiti.
TEMPERATURE of the sea at the Galapagos archipelago.
Tenasserim.
Tenimber island.
Teturoa.
THEORIES on coral-formations.
THEORY OF subsidence, and objections to.
THICKNESS, vertical, of barrier-reefs.
Thomas, St.
Tikopia.
TIMOR, recently elevated.
Timor.
Timor-laut.
Tokan-Bessees.
Tongatabou.
Tonquin.
Toubai.
Toufoa. (Toofoa.)
Toupoua.
TRADITIONS OF CHANGE in coral-islands.
TRIDACNAE embedded in coral-rock. Left exposed in the Low archipelago.
TUBULARIA, quick growth of.
Tumbelan.
Turneffe reef.
Turtle.
Ualan.
VANIKORO, section of. Its state and changes in its reefs.
Vanikoro.
Vine reef.
Virgin Gorda.
Viti archipelago.
VOLCANIC islands, with living corals on their shores. Matter, probably
not associated with thick masses of coral-rock.
VOLCANOES, authorities for their position on the map. Their presence
determined by the movements in progress. Absent or extinct in the areas
of subsidence.
Waigiou.
Wallis island.
Washington.
Well’s reef.
WELLSTEAD, Lieutenant, account of a ship coated with corals.
WEST INDIES, banks of sediment fringed by reefs. Recently elevated.
West Indies.
WHITSUNDAY island, view of. Changes in its state.
WILLIAMS, Rev. J., on traditions of the natives regarding
coral-islands. On antiquity of certain corals.
Wolchonsky.
Wostock.
Xulla islands.
York island.
Yucutan, coast of.
ZONES of different kinds of corals outside the same reefs.
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