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
marginal reef, such as bounds every atoll of the ordinary
form. I cannot refrain from once again remarking on the
singularity of these complex structures - a great sandy and
generally concave disk rises abruptly from the unfathomable
ocean, with its central expanse studded, and its edge
symmetrically bordered with oval basins of coral-rock just
lipping the surface of the sea, sometimes clothed with
vegetation, and each containing a lake of clear water!
One more point in detail:
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Words from 196033 to 196534
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