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 destruction could not have been adduced.
Thus have we traced the history of these great rings of
coral-rock, from their first origin through their normal
changes, and through the occasional accidents of their
existence, to their death and final obliteration.
In my volume on "Coral Formations" I have published a
map, in which I have coloured all the atolls dark-blue, the
barrier-reefs pale-blue, and the fringing reefs red. These
latter reefs have been formed whilst the land has been
stationary, or, as appears from the frequent presence of
upraised organic remains, whilst it has been slowly rising:
atolls and barrier-reefs, on the other hand, have grown up
during the directly opposite movement of subsidence, which
movement must have been very gradual, and in the case of atolls
so vast in amount as to have buried every mountain-summit over
wide ocean-spaces.
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