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_st._—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![1]
[1] These plants are described in the _Annals of Nat. Hist._ vol. i.
1838, p. 337.
In Holman’s[2] _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. “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 N.W. monsoon to the
coast of New Holland, and thence to these islands by the S.E.
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.
[2] Holman’s _Travels_, vol. iv, p. 378.
Chamisso,[3] 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.” 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.
[3] Kotzebue’s _First Voyage_, vol. iii, p. 155.
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.[4] 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[5] the natural history of a lagoon-island in the Radack
Archipelago; 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.
[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 Diopæa, and a
Pterophorus (?); _Diptera_, two species.
[5] Kotzebue’s _First Voyage_, vol. iii, p. 222.
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,[6] carrying on their backs the shells
which they have stolen from the neighbouring beach. 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.
[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.
_Sunday, April_ 3_rd._—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_th._—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,[7]—and the slowness of the
currents of the open sea, are all considered, the occurrence of pebbles
thus transported does appear wonderful. 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.
[7] Some natives carried by Kotzebue to Kamtschatka collected stones
to take back to their country.
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 branchiæ.
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[8] the task would be very
much easier. I was told by Mr. Liesk that on these islands the Birgos
lives only on the nuts which have fallen to the ground.
[8] See _Proceedings of the Zoological Society,_ 1832, p. 17.
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[9] to inhabit a single coral island north of the Society group.
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!
[9] Tyerman and Bennett, _Voyage,_ etc., vol. ii, p. 33.
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
Holuthuriæ (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 holuthuriæ, 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_th._—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,[10] 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. 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.
[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.
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[11] on their formation. 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_, 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.
[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._
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[12] 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.
[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.
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
accompanying sketch 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.
Sections of barrier-reefs
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.
Section of coral-reef
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.
Section of coral-reef
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.
Let us take our new encircling barrier-reef, 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,[13] 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.
[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.
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.
Corals
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.
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, and as so many conditions
before enumerated must affect their existence, it would be an
inexplicable fact if, during the changes to which earth, air, and water
are subjected, the reef-building corals were to keep alive for
perpetuity on any one spot or area. And as by our theory the areas
including atolls and barrier-reefs are subsiding, we ought occasionally
to find reefs both dead and submerged. In all reefs, owing to the
sediment being washed out of the lagoon or lagoon-channel to leeward,
that side is least favourable to the long-continued vigorous growth of
the corals; hence dead portions of reef not unfrequently occur on the
leeward side; and these, though still retaining their proper wall-like
form, are now in several instances sunk several fathoms beneath the
surface. The Chagos group appears from some cause, possibly from the
subsidence having been too rapid, at present to be much less favourably
circumstanced for the growth of reefs than formerly: one atoll has a
portion of its marginal reef, nine miles in length, dead and submerged;
a second has only a few quite small living points which rise to the
surface, a third and fourth are entirely dead and submerged; a fifth is
a mere wreck, with its structure almost obliterated. It is remarkable
that in all these cases the dead reefs and portions of reef
lie at nearly the same depth, namely, from six to eight fathoms beneath
the surface, as if they had been carried down by one uniform movement.
One of these “half-drowned atolls,” so called by Captain Moresby (to
whom I am indebted for much invaluable information), is of vast size,
namely, ninety nautical miles across in one direction, and seventy
miles in another line; and is in many respects eminently curious. As by
our theory it follows that new atolls will generally be formed in each
new area of subsidence, two weighty objections might have been raised,
namely, that atolls must be increasing indefinitely in number; and
secondly, that in old areas of subsidence each separate atoll must be
increasing indefinitely in thickness, if proofs of their occasional
destruction could not have been adduced. Thus have we traced the
history of these great rings of coral-rock, from their first origin
through their normal changes, and through the occasional accidents of
their existence, to their death and final obliteration.
In my volume on _Coral Formations_ I have published a map, in which I
have coloured all the atolls dark-blue, the barrier-reefs pale-blue,
and the fringing reefs red. These latter reefs have been formed whilst
the land has been stationary, or, as appears from the frequent presence
of upraised organic remains, whilst it has been slowly rising: atolls
and barrier-reefs, on the other hand, have grown up during the directly
opposite movement of subsidence, which movement must have been very
gradual, and in the case of atolls so vast in amount as to have buried
every mountain-summit over wide ocean-spaces. Now in this map we see
that the reefs tinted pale and dark-blue, which have been produced by
the same order of movement, as a general rule manifestly stand near
each other. Again we see that the areas with the two blue tints are of
wide extent; and that they lie separate from extensive lines of coast
coloured red, both of which circumstances might naturally have been
inferred, on the theory of the nature of the reefs having been governed
by the nature of the earth’s movement. It deserves notice that in more
than one instance where single red and blue circles approach near each
other, I can show that there have been oscillations of level; for in
such cases the red or fringed
circles consist of atolls, originally by our theory formed during
subsidence, but subsequently upheaved; and on the other hand, some of
the pale-blue or encircled islands are composed of coral-rock, which
must have been uplifted to its present height before that subsidence
took place, during which the existing barrier-reefs grew upwards.
Authors have noticed with surprise that although atolls are the
commonest coral-structures throughout some enormous oceanic tracts,
they are entirely absent in other seas, as in the West Indies: we can
now at once perceive the cause, for where there has not been
subsidence, atolls cannot have been formed; and in the case of the West
Indies and parts of the East Indies, these tracts are known to have
been rising within the recent period. The larger areas, coloured red
and blue, are all elongated; and between the two colours there is a
degree of rude alternation, as if the rising of one had balanced the
sinking of the other. Taking into consideration the proofs of recent
elevation both on the fringed coasts and on some others (for instance,
in South America) where there are no reefs, we are led to conclude that
the great continents are for the most part rising areas: and from the
nature of the coral-reefs, that the central parts of the great oceans
are sinking areas. The East Indian Archipelago, the most broken land in
the world, is in most parts an area of elevation, but surrounded and
penetrated, probably in more lines than one, by narrow areas of
subsidence.
I have marked with vermilion spots all the many known active volcanos
within the limits of this same map. Their entire absence from every one
of the great subsiding areas, coloured either pale or dark blue, is
most striking; and not less so is the coincidence of the chief volcanic
chains with the parts coloured red, which we are led to conclude have
either long remained stationary, or more generally have been recently
upraised. Although a few of the vermilion spots occur within no great
distance of single circles tinted blue, yet not one single active
volcano is situated within several hundred miles of an archipelago, or
even small group of atolls. It is, therefore, a striking fact that in
the Friendly Archipelago, which consists of a group of atolls upheaved
and since partially worn down, two volcanos, and perhaps more, are
historically
known to have been in action. On the other hand, although most of the
islands in the Pacific which are encircled by barrier-reefs are of
volcanic origin, often with the remnants of craters still
distinguishable, not one of them is known to have ever been in
eruption. Hence in these cases it would appear that volcanos burst
forth into action and become extinguished on the same spots,
accordingly as elevatory or subsiding movements prevail there.
Numberless facts could be adduced to prove that upraised organic
remains are common wherever there are active volcanos; but until it
could be shown that in areas of subsidence volcanos were either absent
or inactive, the inference, however probable in itself, that their
distribution depended on the rising or falling of the earth’s surface,
would have been hazardous. But now, I think, we may freely admit this
important deduction.
Taking a final view of the map, and bearing in mind the statements made
with respect to the upraised organic remains, we must feel astonished
at the vastness of the areas which have suffered changes in level
either downwards or upwards, within a period not geologically remote.
It would appear also that the elevatory and subsiding movements follow
nearly the same laws. Throughout the spaces interspersed with atolls,
where not a single peak of high land has been left above the level of
the sea, the sinking must have been immense in amount. The sinking,
moreover, whether continuous, or recurrent with intervals sufficiently
long for the corals again to bring up their living edifices to the
surface, must necessarily have been extremely slow. This conclusion is
probably the most important one which can be deduced from the study of
coral formations;—and it is one which it is difficult to imagine how
otherwise could ever have been arrived at. Nor can I quite pass over
the probability of the former existence of large archipelagoes of lofty
islands, where now only rings of coral-rock scarcely break the open
expanse of the sea, throwing some light on the distribution of the
inhabitants of the other high islands, now left standing so immensely
remote from each other in the midst of the great oceans. The
reef-constructing corals have indeed reared and preserved wonderful
memorials of the subterranean oscillations of level; we see in each
barrier-reef a proof that the land has
there subsided, and in each atoll a monument over an island now lost.
We may thus, like unto a geologist who had lived his ten thousand years
and kept a record of the passing changes, gain some insight into the
great system by which the surface of this globe has been broken up, and
land and water interchanged.
[Illustration: Birgos Latro, Keeling Island]
[Illustration: St. Louis, Mauritius]