ocean, the fury of the breakers, contrasted with the lowness of the
land and the smoothness of the bright green water within the lagoon,
can hardly be imagined without having been seen.
The earlier voyagers fancied that the coral-building animals
instinctively built up their great circles to afford themselves
protection in the inner parts; but so far is this from the truth, that
those massive kinds, to whose growth on the exposed outer shores the
very existence of the reef depends, cannot live within the lagoon,
where other delicately-branching kinds flourish. Moreover, on this
view, many species of distinct genera and families are supposed to
combine for one end; and of such a combination, not a single instance
can be found in the whole of nature. The theory that has been most
generally received is, that atolls are based on submarine craters; but
when we consider the form and size of some, the number, proximity, and
relative positions of others, this idea loses its plausible character:
thus Suadiva atoll is 44 geographical miles in diameter in one line, by
34 miles in another line; Rimsky is 54 by 20 miles across, and it has a
strangely sinuous margin; Bow atoll is 30 miles long, and on an average
only 6 in width; Menchicoff atoll consists of three atolls united or
tied together. This theory, moreover, is totally inapplicable to the
northern Maldiva atolls in the Indian Ocean (one of which is 88 miles
in length, and between 10 and 20 in breadth), for they are not bounded
like ordinary atolls by narrow reefs, but by a vast number of separate
little atolls; other little atolls rising out of the great central
lagoon-like spaces. A third and better theory was advanced by
Chamisso, who thought that from the corals growing more vigorously
where exposed to the open sea, as undoubtedly is the case, the outer
edges would grow up from the general foundation before any other part,
and that this would account for the ring or cup-shaped structure. But
we shall immediately see, that in this, as well as in the
crater-theory, a most important consideration has been overlooked,
namely, on what have the reef-building corals, which cannot live at a
great depth, based their massive structures?
Numerous soundings were carefully taken by Captain Fitz Roy on the
steep outside of Keeling atoll, and it was found that within ten
fathoms, the prepared tallow at the bottom of the lead, invariably came
up marked with the impression of living corals, but as perfectly clean
as if it had been dropped on a carpet of turf; as the depth increased,
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