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 or
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.
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