One of these "half-drowned
atolls," so called by Captain 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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