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
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