uld
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, covering 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.

But how is it possible that the relative level of the land and sea
should be altered to this extent? Clearly,