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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