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.
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