amount of sediment accumulated in it, and to the growth of the
delicately branched corals which can live there. The section in this
state resembles in every respect one drawn through an encircled island:
in fact, it is a real section (on the scale of .517 of an inch to a
mile) through Bolabola in the Pacific. We can now at once see why
encircling barrier-reefs stand so far from the shores which they front.
We can also perceive, that a line drawn perpendicularly down from the
outer edge of the new reef, to the foundation of solid rock beneath the
old fringing-reef, will exceed by as many feet as there have been feet
of subsidence, that small limit of depth at which the effective corals
can live:--the little architects having built up their great wall-like
mass, as the whole sank down, upon a basis formed of other corals and
their consolidated fragments. Thus the difficulty on this head, which
appeared so great, disappears.
If, instead of an island, we had taken the shore of a continent fringed
with reefs, and had imagined it to have subsided, a great straight
barrier, like that of Australia or New Caledonia, separated from the
land by a wide and deep channel, would evidently have been the result.
Let us take our new encircling barrier-reef, of which the section is
now represented by unbroken lines, and which, as I have said, is a real
section through Bolabola, and let it go on subsiding. As the
barrier-reef slowly sinks down, the corals will go on vigorously
growing upwards; but as the island sinks, the water will gain inch by
inch on the shore--the separate mountains first forming separate
islands within
[picture]
one great reef--and finally, the last and highest pinnacle
disappearing. The instant this takes place, a perfect atoll is formed:
I have said, remove the high land from within an encircling
barrier-reef, and an atoll is left, and the land has been removed. We
can now perceive how it comes that atolls, having sprung from
encircling barrier-reefs, resemble them in general size, form, in the
manner in which they are grouped together, and in their arrangement in
single or double lines; for they may be called rude outline charts of
the sunken islands over which they stand. We can further see how it
arises that the atolls in the Pacific and Indian Oceans extend in lines
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