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. 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. And in this same archipelago, Mahlos-Mahdoo atoll
is divided by a bifurcating channel from 100 to 132 fathoms in depth,
in such a manner, that it is scarcely possible to say whether it ought
strictly to be called three separate atolls, or one great atoll not yet
finally divided.
I will not enter on many more details; but I must remark that the
curious structure of the northern Maldiva atolls receives (taking into
consideration the free entrance of the sea through their broken
margins) a simple explanation in the upward and outward growth of the
corals, originally based both on small detached reefs in their lagoons,
such as occur in common atolls, and on broken portions of the linear
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