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 _débris_ 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, covering more and more of the
land, would occupy a wider space between the edge of the reef and what
remained of the land. Thus the rising sea would eventually convert a
large island with a fringing reef, into a small island surrounded by
an encircling reef. And it will be obvious that when the rising of the
sea has gone so far as completely to cover the highest points of the
island, the reef will have passed into the condition of an atoll.