marginal reef, such as bounds every atoll of the ordinary form. I
cannot 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!
One more point in detail: as in the two neighbouring archipelagoes
corals flourish in one and not in the other, and as so many conditions
before enumerated must affect their existence, it would be an
inexplicable fact if, during the changes to which earth, air, and water
are subjected, the reef-building corals were to keep alive for
perpetuity on any one spot or area. And as by our theory the areas
including atolls and barrier-reefs are subsiding, we ought occasionally
to find reefs both dead and submerged. In all reefs, owing to the
sediment being washed out of the lagoon-channel to leeward, that side
is least favourable to the long-continued vigorous growth of the
corals; hence dead portions of reef not unfrequently occur on the
leeward side; and these, though still retaining their proper wall-like
form, are now in several instances sunk several fathoms beneath the
surface. The Chagos group appears from some cause, possibly from the
subsidence having been too rapid, at present to be much less favourably
circumstanced for the growth of reefs than formerly: one atoll has a
portion of its marginal reef, nine miles in length, dead and submerged;
a second has only a few quite small living points which rise to the
surface, a third and fourth are entirely dead and submerged; a fifth is
a mere wreck, with its structure almost obliterated. It is remarkable
that in all these cases, the dead reefs and portions of reef lie at
nearly the same depth, namely, from six to eight fathoms beneath the
surface, as if they had been carried down by one uniform movement. One
of these "half-drowned atolls," so called by Capt. Moresby (to whom I
am indebted for much invaluable information), is of vast size, namely,
ninety nautical miles across in one direction, and seventy miles in
another line; and is in many respects eminently curious. As by our
theory it follows that new atolls will generally be formed in each new
area of subsidence, two weighty objections might have been raised,
namely, that atolls must be increasing indefinitely in number; and
secondly, that in old areas of subsidence each separate atoll must be
increasing indefinitely in thickness, if proofs of their occasional
448