wonderful accumulations of coral rock in the world. It is about 1,100
miles long, and varies in width from one or two to many miles. It is
separated from the coast of Australia by a channel of about 25 fathoms
deep; while outside, looking toward America, the water is two or three
thousand feet deep at a mile from the edge of the reef. This is an
accumulation of limestone rock, built up by corals, to which we have no
parallel anywhere else. Imagine to yourself a heap of this material
more than one thousand miles long, and several miles wide. That is a
barrier reef; but a barrier reef is merely as it were a fragment of an
encircling reef running parallel to the coast of a great continent.

I told you that the polypes which built these reefs were not able to
live at a greater depth than 20 to 25 fathoms of water; and that is the
reason why the fringing reef goes no farther from the land than it
does. And for the same reason, if the Pacific could be laid bare we
should have a most singular spectacle. There would be a number of
mountains with truncated tops scattered over it, and those mountains
would have an appearance just the very reverse of that presented by the
mountains we see on shore. You know that the mountains on shore are
covered with vegetation at their bases, while their tops are barren or
covered with snow; but these mountains would be perfectly bare at their
bases, and all round their tops they would be covered with a beautiful
vegetation of coral polypes. And not only would this be the case, but
we should find that for a considerable distance down, all the material
of these atoll and encircling reefs was built up of precisely the same
coral rock as the fringing reef. That is to say, you have an enormous
mass of coral rock at a depth below the surface of the water where we
know perfectly well that the coral animals could not have lived to form
it. When those two facts were first put together, naturalists were
quite as much puzzled as I daresay you are, at present, to understand