coral-reefs: namely, Atolls, Barrier, and Fringing-Reefs, and will
explain my views 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
meruille de voir chacun de ces atollons, enuironné d’un grand banc de
pierre tout autour, n’y avant point d’artifice humain.” 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, can not 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 Maldive atoll 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 can not 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 4,000 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 1,500 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.
[Illustration: Matterhorn, Valais Alps, Switzerland]
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.
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 can not 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 can not 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.
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 further, 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 inward,
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.
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 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
favorable 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. 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. We can now see why encircling
barrier-reefs stand so far from the shores which they front. We can
also perceive, that a line drawn perpendicularly down from the outer
edge of the new reef, to the foundation of solid rock beneath the
old fringing-reef, will exceed, by as many feet as there have been
feet of subsidence, that small limit of depth at which the effective
corals can live: the little architects having built up their great
wall-like mass, as the whole sank down, upon a basis formed of other
corals and their consolidated fragments. Thus the difficulty on this
head, which appeared so great, disappears.
If, instead of an island, we had taken the shore of a continent
fringed with reefs, and had imagined it to have subsided, a great
straight barrier, like that of Australia or New Caledonia, separated
from the land by a wide and deep channel, would evidently have been
the result.
As the barrier-reef slowly sinks down, the corals will go on
vigorously growing upward; but as the island sinks, the water will
gain inch by inch on the shore--the separate mountains first forming
separate islands within one great reef--and finally, the last and
highest pinnacle disappearing. The instant this takes place, a
perfect atoll is formed: I have said, remove the high land from
within an encircling barrier-reef, and an atoll is left, and the land
has been removed.
We can now perceive how it comes that atolls, having sprung from
encircling barrier-reefs, resemble them in general size, form, in the
manner in which they are grouped together, and in their arrangement
in single or double lines; for they may be called rude outline charts
of the sunken islands over which they stand. We can further see how
it arises that the atolls in the Pacific and Indian Oceans extend in
lines parallel to the generally prevailing strike of the high islands
and great coast-lines of those oceans. I venture, therefore, to
affirm that, on the theory of the upward growth of the corals during
the sinking of the land, all the leading features in those wonderful
structures, the lagoon-islands or atolls, which have so long excited
the attention of voyagers, as well as in the no less wonderful
barrier-reefs, whether encircling small islands or stretching for
hundreds of miles along the shores of a continent, are simply
explained.
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 cocoanut
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 upward: 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
Maldive 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.
Not only the grand features in the structure of barrier-reefs and
of atolls, and of their likeness to each other in form, size, and
other characters, are explained on the theory of subsidence--which
theory we are independently forced to admit in the very areas in
question, from the necessity of finding bases for the corals within
the requisite depth--but many details in structure and exceptional
cases can thus also be simply explained. I will give only a few
instances. In barrier-reefs it has long been remarked with surprise
that the passages through the reef exactly face valleys in the
included land, even in cases where the reef is separated from the
land by a lagoon-channel so wide and so much deeper than the actual
passage itself that it seems hardly possible that the very small
quantity of water or sediment brought down could injure the corals
on the reef. Now, every reef of the fringing class is breached by
a narrow gateway in front of the smallest rivulet, even if dry
during the greater part of the year, for the mud, sand, or gravel,
occasionally washed down, kills the corals on which it is deposited.
Consequently, when an island thus fringed subsides, though most of
the narrow gateways will probably become closed by the outward and
upward growth of the corals, yet any that are not closed (and some
must always be kept open by the sediment and impure water flowing out
of the lagoon-channel) will still continue to front exactly the upper
parts of those valleys at the mouths of which the original basal
fringing-reef was breached.
We can easily see how an island fronted only on one side, or on
one side with one end or both ends encircled by barrier-reefs,
might after long-continued subsidence be converted either into a
single wall-like reef, or into an atoll with a great straight spur
projecting from it, or into two or three atolls tied together by
straight reefs--all of which exceptional cases actually occur. As the
reef-building corals require food, are preyed upon by other animals,
are killed by sediment, can not adhere to a loose bottom, and may be
easily carried down to a depth whence they can not 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 Maldive 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 rims: if
they did not, as the whole sank downward one atoll would be divided
into two or more. In the Maldive 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 Maldive 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 can not 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!