ted 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 how these
two seeming contradictions could be reconciled; and all sorts of odd
hypotheses were resorted to. It was supposed that the coral did not
extend so far down, but that there was a great chain of submarine
mountains stretching through the Pacific, and that the coral had grown
upon them. But only fancy what supposition that was, for you would have
to imagine that there was a chain of mountains a thousand miles or more
long, and that the top of every mountain came within 20 fathoms of the
surface of the sea, and neither rose above nor sunk beneath that level.
That is highly