s no escape from one of these two alternatives. Now the
objections to the notion of the sea having gone up are very considerable
indeed; for you will readily perceive that the sea could not possibly
have risen a thousand feet in the Pacific without rising pretty much
the same distance everywhere else; and if it had risen that height
everywhere else since the reefs began to be formed, the geography of
the world in general must have been very different indeed, at that time,
from what it is now. And we have very good means of knowing that any
such rise as this certainly has not taken place in the level of the sea
since the time that the corals have been building their houses. And so
the only other alternative was to suppose that the land had gone down,
and at so slow a rate that the corals were able to grow upward as fast
as it went downward. 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. 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