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 to their likeness to each other in form,
size, and other characters, are explained on the theory of
subsidence - which theory we are independently forced to
admit in the very areas in question, from the necessity of
finding bases for the corals within the requisite depth - but
many details in structure and exceptional cases can thus also
be simply explained. I will give only a few instances. In
barrier-reefs it has long been remarked with surprise, that
the passages through the reef exactly face valleys in the
included land, even in cases where the reef is separated
from the land by a lagoon-channel so wide and so much
deeper than the actual passage itself, that it seems hardly
possible that the very small quantity of water or sediment
brought down could injure the corals on the reef. Now,
every reef of the fringing class is breached by a narrow
gateway in front of the smallest rivulet, even if dry during
the greater part of the year, for the mud, sand, or gravel,
occasionally washed down kills the corals on which it is
deposited. Consequently, when an island thus fringed subsides,
though most of the narrow gateways will probably
become closed by the outward and upward growth of the
corals, yet any that are not closed (and some must always be
kept open by the sediment and impure water flowing out of
the lagoon-channel) will still continue to front exactly the
upper parts of those valleys, at the mouths of which the
original basal fringing-reef was breached.
We can easily see how an island fronted only on one side, or on
one side with one end or both ends encircled by barrier-reefs,
might after long-continued subsidence be converted
either into a single wall-like reef, or into an atoll with a
great straight spur projecting from it, or into two or three
atolls tied together by straight reefs - all of which
exceptional cases actually occur. As the reef-building corals
require food, are preyed upon by other animals, are killed by
sediment, cannot adhere to a loose bottom, and may be easily
carried down to a depth whence they cannot spring up again,
we need feel no surprise at the reefs both of atolls and
barriers becoming in parts imperfect. The great barrier of
New Caledonia is thus imperfect and broken in many parts;
hence, after long subsidence, this great reef would not produce
one great atoll 400 miles in length, but a chain or
archipelago of atolls, of very nearly the same dimension 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.
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