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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, environne 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



Online LibraryCharles DarwinThe Voyage of the Beagle → online text (page 46 of 51)