obvious when you come to think about it. Suppose we start with a coral
sea and put in the middle of it an island such as the Mauritius. Now
let the coral polypes come and perch on the shore and build a fringing
reef, which will stop when they come to 20 or 25 fathoms, and you will
have a fringing reef like that round the island in the illustration. So
long as the land remains stationary, so long as it does not descend so
long will that reef be unable to get any further out, because the
moment the polype embryos try to get below they die. But now suppose
that the land sinks very gradually indeed. Let it subside by slow
degrees, until the mountain peak, which we have in the middle of it,
alone projects beyond the sea level. The fringing reef would be
carried down also; but we suppose that the sinking is so slow that the
coral polypes are able to grow up as fast as the land is carried down;
consequently they will add layer upon layer until they form a deep cup,
because the inner part of the reef grows much more slowly than the
outer part. Thus you have the reef forming a bed thicker upon the
flanks of the island; but the edge of the reef will be very much further
out from the land, and the lagoon will be many times deeper; in short,
your fringing reef will be converted into an encircling reef. And if,
instead of this being an island, it were a great continent like
Australia, then you will have the phenomenon of a barrier reef which I
have described. The barrier reef of Australia was originally a
fringing reef; the land has gone slowly down; the consequence is the
lagoon has deepened until its depth is now 25 fathoms and the corals
have grown up at the outer edge until you have that prodigious
accumulation which forms the barrier reef at present. Now let this
process go on further still; let us take the land a further step down,
so as to submerge even the peak. The coral, still growing up, will
cover the surface of the land, and you will have an atoll reef; that is
to say, a more or less circular or oval ring of coral rock with a