publication, in the year 1840, of Mr. Darwin's famous work on
coral reefs; 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 150 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, would
be by the gradual accumulation, at the foot of its seaward face, of a
talus of coral fragments torn off by the violence of the waves, which
talus might, in course of time, become high enough to bring its upper
surface within the limits of coral growth, and in that manner provide
a sort of factitious sea-bottom upon which the coral embryos might
perch. If, on the other hand, the level of the sea were slowly and
gradually lowered, it is clear that the parts of its bottom originally