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. Darwin's famous work on coral reefs;[123] in
which a key was given to all the difficult problems connected with the
subject, and every difficulty was shown to be capable of solution by
deductive reasoning from a happy combination of certain well-established
geological and biological truths. Mr. Darwin, in fact, showed that,
so long as the level of the sea remains unaltered in any area in which
coral reefs are being formed, or if the level of the sea relatively
to that of the land is falling, the only reefs which can be formed
are fringing reefs. While if, on the contrary, the level of the sea is
rising relatively to that of the land, at a rate not faster than that
at which the upward growth of the coral can keep pace with it, the reef
will gradually pass from the condition of a fringing, into that of an
encircling or barrier reef. And, finally, that if the relative level of
the sea rise so much that the encircled land is completely submerged,
the reef must necessarily pass into the condition of an atoll.

For, suppose the relative level of the sea to remain stationary, after a
fringing reef has reached that distance from the land at which the depth
of water amounts to one hundred and fifty feet. Then the reef cannot
extend seaward by the migration of coral germs, because these coral
germs would find the bottom of the sea to be too deep for them to live
in. And the only manner in which the reef could extend outwards, wo