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Chapter XX
(PLATE 92. INSIDE AN ATOLL, KEELING ISLAND.)
KEELING ISLAND:--CORAL FORMATIONS.
Keeling Island.
Singular appearance.
Scanty Flora.
Transport of seeds.
Birds and insects.
Ebbing and flowing springs.
Fields of dead coral.
Stones transported in the roots of trees.
Great crab.
Stinging corals.
Coral-eating fish.
Coral formations.
Lagoon islands or atolls.
Depth at which reef-building corals can live.
Vast areas interspersed with low coral islands.
Subsidence of their foundations.
Barrier reefs.
Fringing reefs.
Conversion of fringing-reefs into barrier-reefs, and into atolls.
Evidence of changes in level.
Breaches in barrier-reefs.
Maldiva atolls; their peculiar structure.
Dead and submerged reefs.
Areas of subsidence and elevation.
Distribution of volcanoes.
Subsidence slow and vast in amount.
APRIL 1, 1836.
We arrived in view of the Keeling or Cocos Islands, situated in the
Indian Ocean, and about six hundred miles distant from the coast of
Sumatra. This is one of the lagoon-islands (or atolls) of coral
formation similar to those in the Low Archipelago which we passed
near. When the ship was in the channel at the entrance, Mr. Liesk,
an English resident, came off in his boat. The history of the
inhabitants of this place, in as few words as possible, is as
follows. About nine years ago, Mr. Hare, a worthless character,
brought from the East Indian archipelago a number of Malay slaves,
which now, including children, amount to more than a hundred.
Shortly afterwards Captain Ross, who had before visited these
islands in his merchant-ship, arrived from England, bringing with
him his family and goods for settlement: along with him came Mr.
Liesk, who had been a mate in his vessel. The Malay slaves soon ran
away from the islet on which Mr. Hare was settled, and joined
Captain Ross's party. Mr. Hare upon this was ultimately obliged to
leave the place.
The Malays are now nominally in a state of freedom, and certainly
are so as far as regards their personal treatment; but in most
other points they are considered as slaves. From their discontented
state, from the repeated removals from islet to islet, and perhaps
also from a little mismanagement, things are not very prosperous.
The island has no domestic quadruped excepting the pig, and the
main vegetable production is the cocoa-nut. The whole prosperity of
the place depends on this tree; the only exports being oil from the
nut, and the nuts themselves, which are taken to Singapore and
Mauritius, where they are chiefly used, when grated, in making
curries. On the cocoa-nut, also, the pigs, which are loaded with
fat, almost entirely subsist, as do the ducks and poultry. Even a
huge land-crab is furnished by nature with the means to open and
feed on this most useful production.
The ring-formed reef of the lagoon-island is surmounted in the
greater part of its length by linear islets. On the northern or
leeward side there is an opening through which vessels can pass to
the anchorage within. On entering, the scene was very curious and
rather pretty; its beauty, however, entirely depended on the
brilliancy of the surrounding colours. The shallow, clear, and
still water of the lagoon, resting in its greater part on white
sand, is, when illumined by a vertical sun, of the most vivid
green. This brilliant expanse, several miles in width, is on all
sides divided, either by a line of snow-white breakers from the
dark heaving waters of the ocean, or from the blue vault of heaven
by the strips of land, crowned by the level tops of the cocoa-nut
trees. As a white cloud here and there affords a pleasing contrast
with the azure sky, so in the lagoon bands of living coral darken
the emerald green water.
The next morning after anchoring I went on shore on Direction
Island. The strip of dry land is only a few hundred yards in width;
on the lagoon side there is a white calcareous beach, the radiation
from which under this sultry climate was very oppressive; and on
the outer coast a solid broad flat of coral-rock served to break
the violence of the open sea. Excepting near the lagoon, where
there is some sand, the land is entirely composed of rounded
fragments of coral. In such a loose, dry, stony soil, the climate
of the intertropical regions alone could produce a vigorous
vegetation. On some of the smaller islets nothing could be more
elegant than the manner in which the young and full-grown cocoa-nut
trees, without destroying each other's symmetry, were mingled into
one wood. A beach of glittering white sand formed a border to these
fairy spots.
I will now give a sketch of the natural history of these islands,
which, from its very paucity, possesses a peculiar interest. The
cocoa-nut tree, at first glance, seems to compose the whole wood;
there are however, five or six other trees. One of these grows to a
very large size, but, from the extreme softness of its wood, is
useless; another sort affords excellent timber for ship-building.
Besides the trees the number of plants is exceedingly limited and
consists of insignificant weeds. In my collection, which includes,
I believe, nearly the perfect Flora, there are twenty species
without reckoning a moss, lichen, and fungus. To this number two
trees must be added; one of which was not in flower, and the other
I only heard of. The latter is a solitary tree of its kind, and
grows near the beach, where, without doubt, the one seed was thrown
up by the waves. A Guilandina also grows on only one of the islets.
I do not include in the above list the sugar-cane, banana, some
other vegetables, fruit-trees, and imported grasses. As the islands
consist entirely of coral, and at one time must have existed as
mere water-washed reefs, all their terrestrial productions must
have been transported here by the waves of the sea. In accordance
with this, the Florula has quite the character of a refuge for the
destitute: Professor Henslow informs me that of the twenty species
nineteen belong to different genera, and these again to no less
than sixteen families! (20/1. These plants are described in the
"Annals of Natural History" volume 1 1838 page 337.)
In Holman's "Travels" an account is given, on the authority of Mr.
A.S. Keating, who resided twelve months on these islands, of the
various seeds and other bodies which have been known to have been
washed on shore. (20/2. Holman's "Travels" volume 4 page 378.)
"Seeds and plants from Sumatra and Java have been driven up by the
surf on the windward side of the islands. Among them have been
found the Kimiri, native of Sumatra and the peninsula of Malacca;
the cocoa-nut of Balci, known by its shape and size; the Dadass,
which is planted by the Malays with the pepper-vine, the latter
entwining round its trunk, and supporting itself by the prickles on
its stem; the soap-tree; the castor-oil plant; trunks of the sago
palm; and various kinds of seeds unknown to the Malays settled on
the islands. These are all supposed to have been driven by the
north-west monsoon to the coast of New Holland, and thence to these
islands by the south-east trade-wind. Large masses of Java teak and
Yellow wood have also been found, besides immense trees of red and
white cedar, and the blue gum-wood of New Holland, in a perfectly
sound condition. All the hardy seeds, such as creepers, retain
their germinating power, but the softer kinds, among which is the
mangostin, are destroyed in the passage. Fishing-canoes, apparently
from Java, have at times been washed on shore." It is interesting
thus to discover how numerous the seeds are, which, coming from
several countries, are drifted over the wide ocean. Professor
Henslow tells me he believes that nearly all the plants which I
brought from these islands are common littoral species in the East
Indian archipelago. From the direction, however, of the winds and
currents, it seems scarcely possible that they could have come here
in a direct line. If, as suggested with much probability by Mr.
Keating, they were first carried towards the coast of New Holland,
and thence drifted back together with the productions of that
country, the seeds, before germinating, must have travelled between
1800 and 2400 miles.
Chamisso, when describing the Radack Archipelago, situated in the
western part of the Pacific, states that "the sea brings to these
islands the seeds and fruits of many trees, most of which have yet
not grown here. The greater part of these seeds appear to have not
yet lost the capability of growing." (20/3. Kotzebue's "First
Voyage" volume 3 page 155.) It is also said that palms and bamboos
from somewhere in the torrid zone, and trunks of northern firs, are
washed on shore; these firs must have come from an immense
distance. These facts are highly interesting. It cannot be doubted
that, if there were land-birds to pick up the seeds when first cast
on shore, and a soil better adapted for their growth than the loose
blocks of coral, the most isolated of the lagoon islands would in
time possess a far more abundant Flora than they now have.
The list of land animals is even poorer than that of the plants.
Some of the islets are inhabited by rats, which were brought in a
ship from the Mauritius, wrecked here. These rats are considered by
Mr. Waterhouse as identical with the English kind, but they are
smaller, and more brightly coloured. There are no true land-birds,
for a snipe and a rail (Rallus Phillippensis), though living
entirely in the dry herbage, belong to the order of Waders. Birds
of this order are said to occur on several of the small low islands
in the Pacific. At Ascension, where there is no land-bird, a rail
(Porphyrio simplex) was shot near the summit of the mountain, and
it was evidently a solitary straggler. At Tristan d'Acunha, where,
according to Carmichael, there are only two land-birds, there is a
coot. From these facts I believe that the waders, after the
innumerable web-footed species, are generally the first colonists
of small isolated islands. I may add that whenever I noticed birds,
not of oceanic species, very far out at sea, they always belonged
to this order; and hence they would naturally become the earliest
colonists of any remote point of land.
Of reptiles I saw only one small lizard. Of insects I took pains to
collect every kind. Exclusive of spiders, which were numerous,
there were thirteen species. (20/4. The thirteen species belong to
the following orders:--In the Coleoptera, a minute Elater;
Orthoptera, a Gryllus and a Blatta; Hemiptera, one species;
Homoptera, two; Neuroptera, a Chrysopa; Hymenoptera, two ants;
Lepidoptera nocturna, a Diopaea, and a Pterophorus (?); Diptera,
two species.) Of these one only was a beetle. A small ant swarmed
by thousands under the loose dry blocks of coral, and was the only
true insect which was abundant. Although the productions of the
land are thus scanty, if we look to the waters of the surrounding
sea the number of organic beings is indeed infinite. Chamisso has
described the natural history of a lagoon-island in the Radack
Archipelago (20/5. Kotzebue's "First Voyage" volume 3 page 222.);
and it is remarkable how closely its inhabitants, in number and
kind, resemble those of Keeling Island. There is one lizard and two
waders, namely, a snipe and curlew. Of plants there are nineteen
species, including a fern; and some of these are the same with
those growing here, though on a spot so immensely remote, and in a
different ocean.
The long strips of land, forming the linear islets, have been
raised only to that height to which the surf can throw fragments of
coral, and the wind heap up calcareous sand. The solid flat of
coral rock on the outside, by its breadth, breaks the first
violence of the waves, which otherwise, in a day, would sweep away
these islets and all their productions. The ocean and the land seem
here struggling for mastery: although terra firma has obtained a
footing, the denizens of the water think their claim at least
equally good. In every part one meets hermit crabs of more than one
species, carrying on their backs the shells which they have stolen
from the neighbouring beach. (20/6. The large claws or pincers of
some of these crabs are most beautifully adapted, when drawn back,
to form an operculum to the shell, nearly as perfect as the proper
one originally belonging to the molluscous animal. I was assured,
and as far as my observations went I found it so, that certain
species of the hermit-crab always use certain species of shells.)
Overhead numerous gannets, frigate-birds, and terns, rest on the
trees; and the wood, from the many nests and from the smell of the
atmosphere, might be called a sea-rookery. The gannets, sitting on
their rude nests, gaze at one with a stupid yet angry air. The
noddies, as their name expresses, are silly little creatures. But
there is one charming bird: it is a small, snow-white tern, which
smoothly hovers at the distance of a few feet above one's head, its
large black eye scanning, with quiet curiosity, your expression.
Little imagination is required to fancy that so light and delicate
a body must be tenanted by some wandering fairy spirit.
SUNDAY, APRIL 3, 1836.
After service I accompanied Captain Fitz Roy to the settlement,
situated at the distance of some miles, on the point of an islet
thickly covered with tall cocoa-nut trees. Captain Ross and Mr.
Liesk live in a large barn-like house open at both ends, and lined
with mats made of woven bark. The houses of the Malays are arranged
along the shore of the lagoon. The whole place had rather a
desolate aspect, for there were no gardens to show the signs of
care and cultivation. The natives belong to different islands in
the East Indian archipelago, but all speak the same language: we
saw the inhabitants of Borneo, Celebes, Java, and Sumatra. In
colour they resemble the Tahitians, from whom they do not widely
differ in features. Some of the women, however, show a good deal of
the Chinese character. I liked both their general expressions and
the sound of their voices. They appeared poor, and their houses
were destitute of furniture; but it was evident from the plumpness
of the little children, that cocoa-nuts and turtle afford no bad
sustenance.
On this island the wells are situated from which ships obtain
water. At first sight it appears not a little remarkable that the
fresh water should regularly ebb and flow with the tides; and it
has even been imagined that sand has the power of filtering the
salt from the sea-water. These ebbing wells are common on some of
the low islands in the West Indies. The compressed sand, or porous
coral rock, is permeated like a sponge with the salt water, but the
rain which falls on the surface must sink to the level of the
surrounding sea, and must accumulate there, displacing an equal
bulk of the salt water. As the water in the lower part of the great
sponge-like coral mass rises and falls with the tides, so will the
water near the surface; and this will keep fresh, if the mass be
sufficiently compact to prevent much mechanical admixture; but
where the land consists of great loose blocks of coral with open
interstices, if a well be dug, the water, as I have seen, is
brackish.
After dinner we stayed to see a curious half superstitious scene
acted by the Malay women. A large wooden spoon dressed in garments,
and which had been carried to the grave of a dead man, they pretend
becomes inspired at the full of the moon, and will dance and jump
about. After the proper preparations, the spoon, held by two women,
became convulsed, and danced in good time to the song of the
surrounding children and women. It was a most foolish spectacle;
but Mr. Liesk maintained that many of the Malays believed in its
spiritual movements. The dance did not commence till the moon had
risen, and it was well worth remaining to behold her bright orb so
quietly shining through the long arms of the cocoa-nut trees as
they waved in the evening breeze. These scenes of the tropics are
in themselves so delicious that they almost equal those dearer ones
at home, to which we are bound by each best feeling of the mind.
The next day I employed myself in examining the very interesting,
yet simple structure and origin of these islands. The water being
unusually smooth, I waded over the outer flat of dead rock as far
as the living mounds of coral, on which the swell of the open sea
breaks. In some of the gullies and hollows there were beautiful
green and other coloured fishes, and the form and tints of many of
the zoophytes were admirable. It is excusable to grow enthusiastic
over the infinite numbers of organic beings with which the sea of
the tropics, so prodigal of life, teems; yet I must confess I think
those naturalists who have described, in well-known words, the
submarine grottoes decked with a thousand beauties, have indulged
in rather exuberant language.
APRIL 6, 1836.
I accompanied Captain Fitz Roy to an island at the head of the
lagoon: the channel was exceedingly intricate, winding through
fields of delicately branched corals. We saw several turtle and two
boats were then employed in catching them. The water was so clear
and shallow, that although at first a turtle quickly dives out of
sight, yet in a canoe or boat under sail the pursuers after no very
long chase come up to it. A man standing ready in the bow at this
moment dashes through the water upon the turtle's back; then
clinging with both hands by the shell of its neck, he is carried
away till the animal becomes exhausted and is secured. It was quite
an interesting chase to see the two boats thus doubling about, and
the men dashing head foremost into the water trying to seize their
prey. Captain Moresby informs me that in the Chagos archipelago in
this same ocean, the natives, by a horrible process, take the shell
from the back of the living turtle. "It is covered with burning
charcoal, which causes the outer shell to curl upwards, it is then
forced off with a knife, and before it becomes cold flattened
between boards. After this barbarous process the animal is suffered
to regain its native element, where, after a certain time, a new
shell is formed; it is, however, too thin to be of any service, and
the animal always appears languishing and sickly."
When we arrived at the head of the lagoon we crossed a narrow islet
and found a great surf breaking on the windward coast. I can hardly
explain the reason, but there is to my mind much grandeur in the
view of the outer shores of these lagoon-islands. There is a
simplicity in the barrier-like beach, the margin of green bushes
and tall cocoa-nuts, the solid flat of dead coral-rock, strewed
here and there with great loose fragments, and the line of furious
breakers, all rounding away towards either hand. The ocean throwing
its waters over the broad reef appears an invincible, all-powerful
enemy; yet we see it resisted, and even conquered, by means which
at first seem most weak and inefficient. It is not that the ocean
spares the rock of coral; the great fragments scattered over the
reef, and heaped on the beach, whence the tall cocoa-nut springs,
plainly bespeak the unrelenting power of the waves. Nor are any
periods of repose granted. The long swell caused by the gentle but
steady action of the trade-wind, always blowing in one direction
over a wide area, causes breakers, almost equalling in force those
during a gale of wind in the temperate regions, and which never
cease to rage. It is impossible to behold these waves without
feeling a conviction that an island, though built of the hardest
rock, let it be porphyry, granite, or quartz, would ultimately
yield and be demolished by such an irresistible power. Yet these
low, insignificant coral-islets stand and are victorious: for here
another power, as an antagonist, takes part in the contest. The
organic forces separate the atoms of carbonate of lime, one by one,
from the foaming breakers, and unite them into a symmetrical
structure. Let the hurricane tear up its thousand huge fragments;
yet what will that tell against the accumulated labour of myriads
of architects at work night and day, month after month? Thus do we
see the soft and gelatinous body of a polypus, through the agency
of the vital laws, conquering the great mechanical power of the
waves of an ocean which neither the art of man nor the inanimate
works of nature could successfully resist.
We did not return on board till late in the evening, for we stayed
a long time in the lagoon, examining the fields of coral and the
gigantic shells of the chama, into which, if a man were to put his
hand, he would not, as long as the animal lived, be able to
withdraw it. Near the head of the lagoon I was much surprised to
find a wide area, considerably more than a mile square, covered
with a forest of delicately branching corals, which, though
standing upright, were all dead and rotten. At first I was quite at
a loss to understand the cause; afterwards it occurred to me that
it was owing to the following rather curious combination of
circumstances. It should, however, first be stated, that corals are
not able to survive even a short exposure in the air to the sun's
rays, so that their upward limit of growth is determined by that of
lowest water at spring tides. It appears, from some old charts,
that the long island to windward was formerly separated by wide
channels into several islets; this fact is likewise indicated by
the trees being younger on these portions. Under the former
condition of the reef, a strong breeze, by throwing more water over
the barrier, would tend to raise the level of the lagoon. Now it
acts in a directly contrary manner; for the water within the lagoon
not only is not increased by currents from the outside, but is
itself blown outwards by the force of the wind. Hence it is
observed that the tide near the head of the lagoon does not rise so
high during a strong breeze as it does when it is calm. This
difference of level, although no doubt very small, has, I believe,
caused the death of those coral-groves, which under the former and
more open condition of the outer reef had attained the utmost
possible limit of upward growth.
A few miles north of Keeling there is another small atoll, the
lagoon of which is nearly filled up with coral-mud. Captain Ross
found embedded in the conglomerate on the outer coast a
well-rounded fragment of greenstone, rather larger than a man's
head: he and the men with him were so much surprised at this, that
they brought it away and preserved it as a curiosity. The
occurrence of this one stone, where every other particle of matter
is calcareous, certainly is very puzzling. The island has scarcely
ever been visited, nor is it probable that a ship had been wrecked
there. From the absence of any better explanation, I came to the
conclusion that it must have come entangled in the roots of some
large tree: when, however, I considered the great distance from the
nearest land, the combination of chances against a stone thus being
entangled, the tree washed into the sea, floated so far, then
landed safely, and the stone finally so embedded as to allow of its
discovery, I was almost afraid of imagining a means of transport
apparently so improbable. It was therefore with great interest that
I found Chamisso, the justly distinguished naturalist who
accompanied Kotzebue, stating that the inhabitants of the Radack
Archipelago, a group of lagoon islands in the midst of the Pacific,
obtained stones for sharpening their instruments by searching the
roots of trees which are cast upon the beach. It will be evident
that this must have happened several times, since laws have been
established that such stones belong to the chief, and a punishment
is inflicted on any one who attempts to steal them. When the
isolated position of these small islands in the midst of a vast
ocean--their great distance from any land excepting that of coral
formation, attested by the value which the inhabitants, who are
such bold navigators, attach to a stone of any kind--and the
slowness of the currents of the open sea, are all considered, the
occurrence of pebbles thus transported does appear wonderful.
(20/7. Some natives carried by Kotzebue to Kamtschatka collected
stones to take back to their country.) Stones may often be thus
carried; and if the island on which they are stranded is
constructed of any other substance besides coral, they would
scarcely attract attention, and their origin at least would never
be guessed. Moreover, this agency may long escape discovery from
the probability of trees, especially those loaded with stones,
floating beneath the surface. In the channels of Tierra del Fuego
large quantities of drift timber are cast upon the beach, yet it is
extremely rare to meet a tree swimming on the water. These facts
may possibly throw light on single stones, whether angular or
rounded, occasionally found embedded in fine sedimentary masses.
During another day I visited West Islet, on which the vegetation
was perhaps more luxuriant than on any other. The cocoa-nut trees
generally grow separate, but here the young ones flourished beneath
their tall parents, and formed with their long and curved fronds
the most shady arbours. Those alone who have tried it know how
delicious it is to be seated in such shade, and drink the cool
pleasant fluid of the cocoa-nut. In this island there is a large
bay-like space, composed of the finest white sand: it is quite
level and is only covered by the tide at high water; from this
large bay smaller creeks penetrate the surrounding woods. To see a
field of glittering white sand representing water, with the
cocoa-nut trees extending their tall and waving trunks round the
margin, formed a singular and very pretty view.
I have before alluded to a crab which lives on the cocoa-nuts; it
is very common on all parts of the dry land, and grows to a
monstrous size: it is closely allied or identical with the Birgos
latro. The front pair of legs terminate in very strong and heavy
pincers, and the last pair are fitted with others weaker and much
narrower. It would at first be thought quite impossible for a crab
to open a strong cocoa-nut covered with the husk; but Mr. Liesk
assures me that he has repeatedly seen this effected. The crab
begins by tearing the husk, fibre by fibre, and always from that
end under which the three eye-holes are situated; when this is
completed, the crab commences hammering with its heavy claws on one
of the eye-holes till an opening is made. Then turning round its
body, by the aid of its posterior and narrow pair of pincers it
extracts the white albuminous substance. I think this is as curious
a case of instinct as ever I heard of, and likewise of adaptation
in structure between two objects apparently so remote from each
other in the scheme of nature as a crab and a cocoa-nut tree. The
Birgos is diurnal in its habits; but every night it is said to pay
a visit to the sea, no doubt for the purpose of moistening its
branchiae. The young are likewise hatched, and live for some time,
on the coast. These crabs inhabit deep burrows, which they hollow
out beneath the roots of trees; and where they accumulate
surprising quantities of the picked fibres of the cocoa-nut husk,
on which they rest as on a bed. The Malays sometimes take advantage
of this, and collect the fibrous mass to use as junk. These crabs
are very good to eat; moreover, under the tail of the larger ones
there is a mass of fat, which, when melted, sometimes yields as
much as a quart-bottleful of limpid oil. It has been stated by some
authors that the Birgos crawls up the cocoa-nut trees for the
purpose of stealing the nuts: I very much doubt the possibility of
this; but with the Pandanus the task would be very much easier.
(20/8. See "Proceedings of the Zoological Society" 1832 page 17.) I
was told by Mr. Liesk that on these islands the Birgos lives only
on the nuts which have fallen to the ground.
Captain Moresby informs me that this crab inhabits the Chagos and
Seychelle groups, but not the neighbouring Maldiva archipelago. It
formerly abounded at Mauritius, but only a few small ones are now
found there. In the Pacific this species, or one with closely
allied habits, is said to inhabit a single coral island north of
the Society group. (20/9. Tyerman and Bennett "Voyage" etc. volume
2 page 33.) To show the wonderful strength of the front pair of
pincers, I may mention that Captain Moresby confined one in a
strong tin box, which had held biscuits, the lid being secured with
wire; but the crab turned down the edges and escaped. In turning
down the edges it actually punched many small holes quite through
the tin!
I was a good deal surprised by finding two species of coral of the
genus Millepora (M. complanata and alcicornis), possessed of the
power of stinging. The stony branches or plates, when taken fresh
from the water, have a harsh feel and are not slimy, although
possessing a strong and disagreeable smell. The stinging property
seems to vary in different specimens: when a piece was pressed or
rubbed on the tender skin of the face or arm, a pricking sensation
was usually caused, which came on after the interval of a second,
and lasted only for a few minutes. One day, however, by merely
touching my face with one of the branches, pain was instantaneously
caused; it increased as usual after a few seconds, and remaining
sharp for some minutes, was perceptible for half an hour
afterwards. The sensation was as bad as that from a nettle, but
more like that caused by the Physalia or Portuguese man-of-war.
Little red spots were produced on the tender skin of the arm, which
appeared as if they would have formed watery pustules, but did not.
M. Quoy mentions this case of the Millepora; and I have heard of
stinging corals in the West Indies. Many marine animals seem to
have this power of stinging: besides the Portuguese man-of-war,
many jelly-fish, and the Aplysia or sea-slug of the Cape de Verd
Islands, it is stated in the "Voyage of the Astrolabe" that an
Actinia or sea-anemone, as well as a flexible coralline allied to
Sertularia, both possess this means of offence or defence. In the
East Indian sea a stinging sea-weed is said to be found.
Two species of fish, of the genus Scarus, which are common here,
exclusively feed on coral: both are coloured of a splendid
bluish-green, one living invariably in the lagoon, and the other
amongst the outer breakers. Mr. Liesk assured us that he had
repeatedly seen whole shoals grazing with their strong bony jaws on
the tops of the coral branches: I opened the intestines of several
and found them distended with yellowish calcareous sandy mud. The
slimy disgusting Holuthuriae (allied to our star-fish), which the
Chinese gourmands are so fond of, also feed largely, as I am
informed by Dr. Allan, on corals; and the bony apparatus within
their bodies seems well adapted for this end. These holuthuriae,
the fish, the numerous burrowing shells, and nereidous worms, which
perforate every block of dead coral, must be very efficient agents
in producing the fine white mud which lies at the bottom and on the
shores of the lagoon. A portion, however, of this mud, which when
wet resembled pounded chalk, was found by Professor Ehrenberg to be
partly composed of siliceous-shielded infusoria.
APRIL 12, 1836.
In the morning we stood out of the lagoon on our passage to the
Isle of France. I am glad we have visited these islands: such
formations surely rank high amongst the wonderful objects of this
world. Captain Fitz Roy found no bottom with a line 7200 feet in
length, at the distance of only 2200 yards from the shore; hence
this island forms a lofty submarine mountain, with sides steeper
even than those of the most abrupt volcanic cone. The saucer-shaped
summit is nearly ten miles across; and every single atom, from the
least particle to the largest fragment of rock, in this great pile,
which however is small compared with very many other lagoon
islands, bears the stamp of having been subjected to organic
arrangement. (20/10. I exclude, of course, some soil which has been
imported here in vessels from Malacca and Java, and likewise some
small fragments of pumice, drifted here by the waves. The one block
of greenstone, moreover, on the northern island must be excepted.)
We feel surprise when travellers tell us of the vast dimensions of
the Pyramids and other great ruins, but how utterly insignificant
are the greatest of these, when compared to these mountains of
stone accumulated by the agency of various minute and tender
animals! This is a wonder which does not at first strike the eye of
the body, but, after reflection, the eye of reason.
(PLATE 93. WHITSUNDAY ISLAND.)
I will now give a very brief account of the three great classes of
coral-reefs; namely, Atolls, Barrier, and Fringing Reefs, and will
explain my views on their formation. (20/11. These were first read
before the Geological Society in May 1837 and have since been
developed in a separate volume on the "Structure and Distribution
of Coral Reefs.") Almost every voyager who has crossed the Pacific
has expressed his unbounded astonishment at the lagoon islands, or
as I shall for the future call them by their Indian name of atolls,
and has attempted some explanation. Even as long ago as the year
1605, Pyrard de Laval well exclaimed, "C'est une merveille de voir
chacun de ces atollons, environné d'un grand banc de pierre tout
autour, n'y ayant point d'artifice humain." The accompanying sketch
of Whitsunday Island in the Pacific, copied from Captain Beechey's
admirable "Voyage" (Plate 93), gives but a faint idea of the
singular aspect of an atoll: it is one of the smallest size, and
has its narrow islets united together in a ring. The immensity of
the ocean, the fury of the breakers, contrasted with the lowness of
the land and the smoothness of the bright green water within the
lagoon, can hardly be imagined without having been seen.
The earlier voyagers fancied that the coral-building animals
instinctively built up their great circles to afford themselves
protection in the inner parts; but so far is this from the truth
that those massive kinds, to whose growth on the exposed outer
shores the very existence of the reef depends, cannot live within
the lagoon, where other delicately-branching kinds flourish.
Moreover, on this view, many species of distinct genera and
families are supposed to combine for one end; and of such a
combination, not a single instance can be found in the whole of
nature. The theory that has been most generally received is that
atolls are based on submarine craters; but when we consider the
form and size of some, the number, proximity, and relative
positions of others, this idea loses its plausible character: thus
Suadiva atoll is 44 geographical miles in diameter in one line, by
34 miles in another line; Rimsky is 54 by 20 miles across, and it
has a strangely sinuous margin; Bow atoll is 30 miles long, and on
an average only 6 in width; Menchicoff atoll consists of three
atolls united or tied together. This theory, moreover, is totally
inapplicable to the northern Maldiva atolls in the Indian Ocean
(one of which is 88 miles in length, and between 10 and 20 in
breadth), for they are not bounded like ordinary atolls by narrow
reefs, but by a vast number of separate little atolls; other little
atolls rising out of the great central lagoon-like spaces. A third
and better theory was advanced by Chamisso, who thought that from
the corals growing more vigorously where exposed to the open sea,
as undoubtedly is the case, the outer edges would grow up from the
general foundation before any other part, and that this would
account for the ring or cup-shaped structure. But we shall
immediately see, that in this, as well as in the crater-theory, a
most important consideration has been overlooked, namely, on what
have the reef-building corals, which cannot live at a great depth,
based their massive structures?
Numerous soundings were carefully taken by Captain Fitz Roy on the
steep outside of Keeling atoll, and it was found that within ten
fathoms the prepared tallow at the bottom of the lead invariably
came up marked with the impressions of living corals, but as
perfectly clean as if it had been dropped on a carpet of turf; as
the depth increased, the impressions became less numerous, but the
adhering particles of sand more and more numerous, until at last it
was evident that the bottom consisted of a smooth sandy layer; to
carry on the analogy of the turf, the blades of grass grew thinner
and thinner, till at last the soil was so sterile that nothing
sprang from it. From these observations, confirmed by many others,
it may be safely inferred that the utmost depth at which corals can
construct reefs is between 20 and 30 fathoms. Now there are
enormous areas in the Pacific and Indian Oceans in which every
single island is of coral formation, and is raised only to that
height to which the waves can throw up fragments, and the winds
pile up sand. Thus the Radack group of atolls is an irregular
square, 520 miles long and 240 broad; the Low Archipelago is
elliptic-formed, 840 miles in its longer, and 420 in its shorter
axis: there are other small groups and single low islands between
these two archipelagoes, making a linear space of ocean actually
more than 4000 miles in length, in which not one single island
rises above the specified height. Again, in the Indian Ocean there
is a space of ocean 1500 miles in length, including three
archipelagoes, in which every island is low and of coral formation.
From the fact of the reef-building corals not living at great
depths, it is absolutely certain that throughout these vast areas,
wherever there is now an atoll, a foundation must have originally
existed within a depth of from 20 to 30 fathoms from the surface.
It is improbable in the highest degree that broad, lofty, isolated,
steep-sided banks of sediment, arranged in groups and lines
hundreds of leagues in length, could have been deposited in the
central and profoundest parts of the Pacific and Indian Oceans, at
an immense distance from any continent, and where the water is
perfectly limpid. It is equally improbable that the elevatory
forces should have uplifted throughout the above vast areas,
innumerable great rocky banks within 20 to 30 fathoms, or 120 to
180 feet, of the surface of the sea, and not one single point above
that level; for where on the whole face of the globe can we find a
single chain of mountains, even a few hundred miles in length, with
their many summits rising within a few feet of a given level, and
not one pinnacle above it? If then the foundations, whence the
atoll-building corals sprang, were not formed of sediment, and if
they were not lifted up to the required level, they must of
necessity have subsided into it; and this at once solves the
difficulty. For as mountain after mountain, and island after
island, slowly sank beneath the water, fresh bases would be
successively afforded for the growth of the corals. It is
impossible here to enter into all the necessary details, but I
venture to defy any one to explain in any other manner how it is
possible that numerous islands should be distributed throughout
vast areas--all the islands being low--all being built of corals,
absolutely requiring a foundation within a limited depth from the
surface. (20/12. It is remarkable that Mr. Lyell, 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.)
(PLATE 94. BARRIER-REEF, BOLABOLA.)
Before explaining how atoll-formed reefs acquire their peculiar
structure, we must turn to the second great class, namely,
Barrier-reefs. These either extend in straight lines in front of
the shores of a continent or of a large island, or they encircle
smaller islands; in both cases, being separated from the land by a
broad and rather deep channel of water, analogous to the lagoon
within an atoll. It is remarkable how little attention has been
paid to encircling barrier-reefs; yet they are truly wonderful
structures. The sketch (Plate 94) represents part of the barrier
encircling the island of Bolabola in the Pacific, as seen from one
of the central peaks. In this instance the whole line of reef has
been converted into land; but usually a snow-white line of great
breakers, with only here and there a single low islet crowned with
cocoa-nut trees, divides the dark heaving waters of the ocean from
the light green expanse of the lagoon-channel. And the quiet waters
of this channel generally bathe a fringe of low alluvial soil,
loaded with the most beautiful productions of the tropics, and
lying at the foot of the wild, abrupt, central mountains.
Encircling barrier-reefs are of all sizes, from three miles to no
less than forty-four miles in diameter; and that which fronts one
side, and encircles both ends, of New Caledonia, is 400 miles long.
Each reef includes one, two, or several rocky islands of various
heights; and in one instance, even as many as twelve separate
islands. The reef runs at a greater or less distance from the
included land; in the Society Archipelago generally from one to
three or four miles; but at Hogoleu the reef is 20 miles on the
southern side, and 14 miles on the opposite or northern side, from
the included islands. The depth within the lagoon-channel also
varies much; from 10 to 30 fathoms may be taken as an average; but
at Vanikoro there are spaces no less than 56 fathoms or 336 feet
deep. Internally the reef either slopes gently into the
lagoon-channel, or ends in a perpendicular wall sometimes between
two and three hundred feet under water in height: externally the
reef rises, like an atoll, with extreme abruptness out of the
profound depths of the ocean. What can be more singular than these
structures? We see an island, which may be compared to a castle
situated on the summit of a lofty submarine mountain, protected by
a great wall of coral-rock, always steep externally and sometimes
internally, with a broad level summit, here and there breached by
narrow gateways, through which the largest ships can enter the wide
and deep encircling moat.
As far as the actual reef of coral is concerned, there is not the
smallest difference in general size, outline, grouping, and even in
quite trifling details of structure, between a barrier and an
atoll. The geographer Balbi has well remarked that an encircled
island is an atoll with high land rising out of its lagoon; remove
the land from within, and a perfect atoll is left.
But what has caused these reefs to spring up at such great
distances from the shores of the included islands? It cannot be
that the corals will not grow close to the land; for the shores
within the lagoon-channel, when not surrounded by alluvial soil,
are often fringed by living reefs; and we shall presently see that
there is a whole class, which I have called Fringing-reefs from
their close attachment to the shores both of continents and of
islands. Again, on what have the reef-building corals, which cannot
live at great depths, based their encircling structures? This is a
great apparent difficulty, analogous to that in the case of atolls,
which has generally been overlooked. It will be perceived more
clearly by inspecting the following sections which are real ones,
taken in north and south lines, through the islands with their
barrier-reefs, of Vanikoro, Gambier, and Maurua; and they are laid
down, both vertically and horizontally, on the same scale of a
quarter of an inch to a mile.
(PLATE 95. SECTIONS OF BARRIER-REEFS. 1. Vanikoro. 2. Gambier
Islands. 3. Maurua.
The horizontal shading shows the barrier-reefs and lagoon-channels.
The inclined shading above the level of the sea (AA) shows the
actual form of the land; the inclined shading below this line shows
its probable prolongation under water.)
It should be observed that the sections might have been taken in
any direction through these islands, or through many other
encircled islands, and the general features would have been the
same. Now bearing in mind that reef-building coral cannot live at a
greater depth than from 20 to 30 fathoms, and that the scale is so
small that the plummets on the right hand show a depth of 200
fathoms, on what are these barrier-reefs based? Are we to suppose
that each island is surrounded by a collar-like submarine ledge of
rock, or by a great bank of sediment, ending abruptly where the
reef ends? If the sea had formerly eaten deeply into the islands,
before they were protected by the reefs, thus having left a shallow
ledge round them under water, the present shores would have been
invariably bounded by great precipices; but this is most rarely the
case. Moreover, on this notion, it is not possible to explain why
the corals should have sprung up, like a wall, from the extreme
outer margin of the ledge, often leaving a broad space of water
within, too deep for the growth of corals. The accumulation of a
wide bank of sediment all round these islands, and generally widest
where the included islands are smallest, is highly improbable,
considering their exposed positions in the central and deepest
parts of the ocean. In the case of the barrier-reef of New
Caledonia, which extends for 150 miles beyond the northern point of
the island, in the same straight line with which it fronts the west
coast, it is hardly possible to believe that a bank of sediment
could thus have been straightly deposited in front of a lofty
island, and so far beyond its termination in the open sea. Finally,
if we look to other oceanic islands of about the same height and of
similar geological constitution, but not encircled by coral-reefs,
we may in vain search for so trifling a circumambient depth as 30
fathoms, except quite near to their shores; for usually land that
rises abruptly out of water, as do most of the encircled and
non-encircled oceanic islands, plunges abruptly under it. On what
then, I repeat, are these barrier reefs based? Why, with their wide
and deep moat-like channels, do they stand so far from the included
land? We shall soon see how easily these difficulties disappear.
We come now to our third class of Fringing-reefs, which will
require a very short notice. Where the land slopes abruptly under
water, these reefs are only a few yards in width, forming a mere
ribbon or fringe round the shores: where the land slopes gently
under the water the reef extends farther, sometimes even as much as
a mile from the land; but in such cases the soundings outside the
reef always show that the submarine prolongation of the land is
gently inclined. In fact the reefs extend only to that distance
from the shore at which a foundation within the requisite depth
from 20 to 30 fathoms is found. As far as the actual reef is
concerned, there is no essential difference between it and that
forming a barrier or an atoll: it is, however, generally of less
width, and consequently few islets have been formed on it. From the
corals growing more vigorously on the outside, and from the noxious
effect of the sediment washed inwards, the outer edge of the reef
is the highest part, and between it and the land there is generally
a shallow sandy channel a few feet in depth. Where banks of
sediment have accumulated near to the surface, as in parts of the
West Indies, they sometimes become fringed with corals, and hence
in some degree resemble lagoon-islands or atolls, in the same
manner as fringing-reefs, surrounding gently sloping islands, in
some degree resemble barrier-reefs.
(PLATE 96. SECTION OF CORAL-REEF.
AA, Outer edges of the fringing-reef, at the level of the sea. BB,
The shores of the fringed island. A'A', Outer edges of the reef,
after its upward growth during a period of subsidence, now
converted into a barrier, with islets on it. B'B', The shores of
the now encircled islands. CC, Lagoon-channel. NB.--In this and
Plate 97, the subsidence of the land could be represented only by
an apparent rise in the level of the sea.)
No theory on the formation of coral-reefs can be considered
satisfactory which does not include the three great classes. We
have seen that we are driven to believe in the subsidence of those
vast areas, interspersed with low islands, of which not one rises
above the height to which the wind and waves can throw up matter,
and yet are constructed by animals requiring a foundation, and that
foundation to lie at no great depth. Let us then take an island
surrounded by fringing-reefs, which offer no difficulty in their
structure; and let this island with its reef, represented by the
unbroken lines in Plate 96, slowly subside. Now as the island sinks
down, either a few feet at a time or quite insensibly, we may
safely infer, from what is known 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 inner edge of
the reef and the beach proportionally 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 reef; and a ship is anchored in
the lagoon-channel. This channel will be more or less deep,
according to the rate of subsidence, to the amount of sediment
accumulated in it, and to the growth of the delicately branched
corals which can live there. The section in this state resembles in
every respect one drawn through an encircled island: in fact, it is
a real section (on the scale of .517 of an inch to a mile) through
Bolabola in the Pacific. We can now at once see why encircling
barrier-reefs stand so far from the shores which they front. We can
also perceive that a line drawn perpendicularly down from the outer
edge of the new reef, to the foundation of solid rock beneath the
old fringing-reef, will exceed by as many feet as there have been
feet of subsidence, that small limit of depth at which the
effective corals can live:--the little architects having built up
their great wall-like mass, as the whole sank down, upon a basis
formed of other corals and their consolidated fragments. Thus the
difficulty on this head, which appeared so great, disappears.
If, instead of an island, we had taken the shore of a continent
fringed with reefs, and had imagined it to have subsided, a great
straight barrier, like that of Australia or New Caledonia,
separated from the land by a wide and deep channel, would evidently
have been the result.
(PLATE 97. SECTION OF CORAL-REEF. A'A', Outer edges of the
barrier-reef at the level of the sea, with islets on it. B'B', The
shores of the included island. CC, The lagoon-channel. A''A'',
Outer edges of the reef, now converted into an atoll. C', The
lagoon of the new atoll. NB.--According to the true scale, the
depths of the lagoon-channel and lagoon are much exaggerated.)
Let us take our new encircling barrier-reef (Plate 97), of which
the section is now represented by unbroken lines, and which, as I
have said, is a real section through Bolabola, and let it go on
subsiding. As the barrier-reef slowly sinks down, the corals will
go on vigorously growing upwards; but as the island sinks, the
water will gain inch by inch on the shore--the separate mountains
first forming separate islands within one great reef--and finally,
the last and highest pinnacle disappearing. The instant this takes
place, a perfect atoll is formed: I have said, remove the high land
from within an encircling barrier-reef, and an atoll is left, and
the land has been removed. We can now perceive how it comes that
atolls, having sprung from encircling barrier-reefs, resemble them
in general size, form, in the manner in which they are grouped
together, and in their arrangement in single or double lines; for
they may be called rude outline charts of the sunken islands over
which they stand. We can further see how it arises that the atolls
in the Pacific and Indian Oceans extend in lines parallel to the
generally prevailing strike of the high islands and great
coast-lines of those oceans. I venture, therefore, to affirm that
on the theory of the upward growth of the corals during the sinking
of the land, all the leading features in those wonderful
structures, the lagoon-islands or atolls, which have so long
excited the attention of voyagers, as well as in the no less
wonderful barrier-reefs, whether encircling small islands or
stretching for hundreds of miles along the shores of a continent,
are simply explained. (20/13. It has been highly satisfactory to me
to find the following passage in a pamphlet by Mr. Couthouy, one of
the naturalists in the great Antarctic Expedition of the United
States:--"Having personally examined a large number of
coral-islands, and resided 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." The naturalists, however,
of this expedition differ with me on some points respecting coral
formations.)
(PLATE 98. BOLABOLA ISLAND.)
It may be asked whether I can offer any direct evidence of the
subsidence of barrier-reefs or atolls; but it must be borne in mind
how difficult it must ever be to detect a movement, the tendency of
which is to hide under water the part affected. Nevertheless, at
Keeling atoll I observed on all sides of the lagoon old cocoa-nut
trees undermined and falling; and in one place the foundation-posts
of a shed, which the inhabitants asserted had stood seven years
before just above high-water mark, but now was daily washed by
every tide; on inquiry I found that three earthquakes, one of them
severe, had been felt here during the last ten years. At Vanikoro
the lagoon-channel is remarkably deep, scarcely any alluvial soil
has accumulated at the foot of the lofty included mountains, and
remarkably few islets have been formed by the heaping of fragments
and sand on the wall-like barrier reef; these facts, and some
analogous ones, led me to believe that this island must lately have
subsided and the reef grown upwards: here again earthquakes are
frequent and very severe. In the Society Archipelago, on the other
hand, where the lagoon-channels are almost choked up, where much
low alluvial land has accumulated, and where in some cases long
islets have been formed on the barrier-reefs--facts all showing
that the islands have not very lately subsided--only feeble shocks
are most rarely felt. In these coral formations, where the land and
water seem struggling for mastery, it must be ever difficult to
decide between the effects of a change in the set of the tides and
of a slight subsidence: that many of these reefs and atolls are
subject to changes of some kind is certain; on some atolls the
islets appear to have increased greatly within a late period; on
others they have been partially or wholly washed away. The
inhabitants of parts of the Maldiva Archipelago know the date of
the first formation of some islets; in other parts the corals are
now flourishing on water-washed reefs, where holes made for graves
attest the former existence of inhabited land. It is difficult to
believe in frequent changes in the tidal currents of an open ocean;
whereas we have in the earthquakes recorded by the natives on some
atolls, and in the great fissures observed on other atolls, plain
evidence of changes and disturbances in progress in the
subterranean regions.
It is evident, on our theory, that coasts merely fringed by reefs
cannot have subsided to any perceptible amount; and therefore they
must, since the growth of their corals, either have remained
stationary or have been upheaved. Now it is remarkable how
generally it can be shown, by the presence of upraised organic
remains, that the fringed islands have been elevated: and so far,
this is indirect evidence in favour of our theory. I was
particularly struck with this fact, when I found, to my surprise,
that the descriptions given by MM. Quoy and Gaimard were
applicable, not to reefs in general as implied by them, but only to
those of the fringing class; my surprise, however, ceased when I
afterwards found that, by a strange chance, all the several islands
visited by these eminent naturalists could be shown by their own
statements to have been elevated within a recent geological era.
Not only the grand features in the structure of barrier-reefs and
of atolls, and of their likeness to each other in form, size, and
other characters, are explained on the theory of subsidence--which
theory we are independently forced to admit in the very areas in
question, from the necessity of finding bases for the corals within
the requisite depth--but many details in structure and exceptional
cases can thus also be simply explained. I will give only a few
instances. In barrier-reefs it has long been remarked with surprise
that the passages through the reef exactly face valleys in the
included land, even in cases where the reef is separated from the
land by a lagoon-channel so wide and so much deeper than the actual
passage itself, that it seems hardly possible that the very small
quantity of water or sediment brought down could injure the corals
on the reef. Now, every reef of the fringing class is breached by a
narrow gateway in front of the smallest rivulet, even if dry during
the greater part of the year, for the mud, sand, or gravel
occasionally washed down kills the corals on which it is deposited.
Consequently, when an island thus fringed subsides, though most of
the narrow gateways will probably become closed by the outward and
upward growth of the corals, yet any that are not closed (and some
must always be kept open by the sediment and impure water flowing
out of the lagoon-channel) will still continue to front exactly the
upper parts of those valleys at the mouths of which the original
basal fringing-reef was breached.
We can easily see how an island fronted only on one side, or on one
side with one end or both ends encircled by barrier-reefs, might
after long-continued subsidence be converted either into a single
wall-like reef, or into an atoll with a great straight spur
projecting from it, or into two or three atolls tied together by
straight reefs--all of which exceptional cases actually occur. As
the reef-building corals require food, are preyed upon by other
animals, are killed by sediment, cannot adhere to a loose bottom,
and may be easily carried down to a depth whence they cannot spring
up again, we need feel no surprise at the reefs both of atolls and
barriers becoming in parts imperfect. The great barrier of New
Caledonia is thus imperfect and broken in many parts; hence, after
long subsidence, this great reef would not produce one great atoll
400 miles in length, but a chain or archipelago of atolls, of very
nearly the same dimensions with those in the Maldiva Archipelago.
Moreover, in an atoll once breached on opposite sides, from the
likelihood of the oceanic and tidal currents passing straight
through the breaches, it is extremely improbable that the corals,
especially during continued subsidence, would ever be able again to
unite the rim; if they did not, as the whole sank downwards, one
atoll would be divided into two or more. In the Maldiva Archipelago
there are distinct atolls so related to each other in position, and
separated by channels either unfathomable or very deep (the channel
between Ross and Ari atolls is 150 fathoms, and that between the
north and south Nillandoo atolls is 200 fathoms in depth), that it
is impossible to look at a map of them without believing that they
were once more intimately related. And in this same archipelago,
Mahlos-Mahdoo atoll is divided by a bifurcating channel from 100 to
132 fathoms in depth, in such a manner that it is scarcely possible
to say whether it ought strictly to be called three separate
atolls, or one great atoll not yet finally divided.
(PLATE 99. CORALS.)
I will not enter on many more details; but I must remark that the
curious structure of the northern Maldiva atolls receives (taking
into consideration the free entrance of the sea through their
broken margins) a simple explanation in the upward and outward
growth of the corals, originally based both on small detached reefs
in their lagoons, such as occur in common atolls, and on broken
portions of the linear marginal reef, such as bounds every atoll of
the ordinary form. I cannot refrain from once again remarking on
the singularity of these complex structures--a great sandy and
generally concave disk rises abruptly from the unfathomable ocean,
with its central expanse studded and its edge symmetrically
bordered with oval basins of coral-rock just lipping the surface of
the sea, sometimes clothed with vegetation, and each containing a
lake of clear water!
One more point in detail: as in the two neighbouring archipelagoes
corals flourish in one and not in the other, a
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