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 100 or 150 feet, how can they have built up
the base of the reef-cone, which may be 2,000 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 1,000 miles or more, and so nearly of the same height,
that none should rise above the level of the sea, nor fall 150 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