Now,
Every Reef Of The Fringing Class Is Breached By A Narrow
Gateway In Front Of The Smallest Rivulet, Even If Dry During
The Greater Part Of The Year, For The Mud, Sand, Or Gravel,
Occasionally Washed Down Kills The Corals On Which It Is
Deposited.
Consequently, when an island thus fringed subsides,
though most of the narrow gateways will probably
become closed by the
Outward and upward growth of the
corals, yet any that are not closed (and some must always be
kept open by the sediment and impure water flowing out of
the lagoon-channel) will still continue to front exactly the
upper parts of those valleys, at the mouths of which the
original basal fringing-reef was breached.
We can easily see how an island fronted only on one side, or on
one side with one end or both ends encircled by barrier-reefs,
might after long-continued subsidence be converted
either into a single wall-like reef, or into an atoll with a
great straight spur projecting from it, or into two or three
atolls tied together by straight reefs - all of which
exceptional cases actually occur. As the reef-building corals
require food, are preyed upon by other animals, are killed by
sediment, cannot adhere to a loose bottom, and may be easily
carried down to a depth whence they cannot spring up again,
we need feel no surprise at the reefs both of atolls and
barriers becoming in parts imperfect. The great barrier of
New Caledonia is thus imperfect and broken in many parts;
hence, after long subsidence, this great reef would not produce
one great atoll 400 miles in length, but a chain or
archipelago of atolls, of very nearly the same dimension with
those in the Maldiva archipelago.
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