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 dimensions with those in the Maldiva Archipelago.
Moreover, in an atoll once breached on opposite sides, from the
likelihood of the oceanic and tidal currents passing straight
through the breaches, it is extremely improbable that the corals,
especially during continued subsidence, would ever be able again to
unite the rim; if they did not, as the whole sank downwards, one
atoll would be divided into two or more. In the Maldiva Archipelago
there are distinct atolls so related to each other in position, and
separated by channels either unfathomable or very deep (the channel
between Ross and Ari atolls is 150 fathoms, and that between the
north and south Nillandoo atolls is 200 fathoms in depth), that it
is impossible to look at a map of them without believing that they
were once more intimately related.
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