evel 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
beyond the limit of coral growth, would gradually be brought within
the required distance of the surface, and thus the reef might be
indefinitely extended. But this process would give rise neither to an
encircling reef nor to an atoll, but to a broad belt of upheaved
coral rock, increasing the dimensions of the dry land, and continuous
seawards with the fresh fringing reef.

Suppose, however, that the sea-level rose instead of falling, at the
same slow and gradual rate at which we know it to be rising in some
parts of the world--not more, in fact, than a few inches, or, at
most, a foot or two, in a hundred years. Then, while the reef would
be unable to extend itself seaward, the sea-bottom outside it being
gradually more and more removed from the depth at which the life of
the coral polypes is possible, it would be able to grow upwards
as fast as the sea rose. But the growth would take place almost
exclusively around the circumference of the reef, this being the only
region in which the coral polypes would find the conditions favourable
for their existence. The bottom of the lagoon would be raised, in the
main, only by the coral _debris_ and coral mud, formed in the manner
already described; consequently, the margins of the reef would
rise faster than the bottom, or, in other words, the lagoon would
constantly become deeper. And, at the same time, it would gradually
increase in breadth; as the rising sea