For these are white with snow at the top, while their bases are clothed
with an abundant and gaudily-coloured vegetation. But the coral cones
would look grey and barren below, while their summits would be gay with
a richly-coloured parterre of flowerlike coral polypes.
The practical difficulties of sounding upon, and of bringing up portions
of, the seaward face of an atoll or of an encircling reef, are so great,
in consequence of the constant and dangerous swell which sets towards
it, that no exact information concerning the depth to which the reefs
are composed of coral has yet been obtained. There is no reason to
doubt, however, that the reef-cone has the same structure from its
summit to its base, and that its sea-wall is throughout mainly composed
of dead coral.
And now arises a serious difficulty. If the coral polypes cannot live at
a greater depth than one hundred or one hundred and fifty feet, how can
they have built up the base of the reef-cone, which may be two thousand
feet, or more, below the surface of the sea?
In order to get over this objection, it was at one time supposed that
the reef-building polypes had settled upon the summits of a chain of
submarine mountains. But what is there in physical geography to justify
the assumption of the existence of a chain of mountains stretching for
one thousand miles or more, and so nearly of the same height, that none
should rise above the level of the sea, nor fall one hundred and fifty
feet below that level?
How, again, on this hypothesis, are atolls to be accounted for, unless,
as some have done, we take refuge in the wild supposition that every
atoll corresponds with the crater of a submarine volcano? And what
explanation does it afford of the fact that, in some parts of the ocean,
only atolls and encircling reefs occur, while others present none but
fringing reefs?
These and other puzzling facts remained insoluble until the publication,
in the year 1840, of Mr.
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