You will see at once that
this is the solution of the mystery, and nothing can be simpler or more
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.
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