are some of them which remain single, or which give rise to only small
accumulations; and the skeletons of these, as they die, accumulate upon
the bottom of the sea, but they do not come to much; they are washed
about and do not adhere together, but become mixed up with the mud of
the sea. But there are certain parts of the world in which the coral
polypes which live and grow are of a kind which remain, adhere
together, and form great masses. They differ from the ordinary polypes
just in the same way as those plants which form a peat-bog or
meadow-turf differ from ordinary plants. They have a habit of growing
together in masses in the same place; they are what we call
"gregarious" things; and the consequence of this is, that as they die
and leave their skeletons, those skeletons form a considerable solid
aggregation at the bottom of the sea, and other polypes perch upon
them, and begin building upon them, and so by degrees a great mass is
formed. And just as we know there are some ancient cities in which you
have a British city, and over that the foundations of a Roman city; and
over that a Saxon city, and over that again a modern city, so in these
localities of which I am speaking, you have the accumulations of the
foundations of the houses, if I may use the term, of nation after nation
of these coral polypes; and these accumulations may cover a very
considerable space, and may rise in the course of time from the bottom
to the surface of the sea.

Mariners have a name which they apply to all sorts of obstacles
consisting of hard and rocky matter which comes in their way in the
course of their navigation; they call such obstacles "reefs," and they
have long been in the habit of calling the particular kind of reef,
which is formed by the accumulation of the skeletons of dead corals, by
the name of "coral reefs," therefore, those parts of the world in which
these accumulations occur have been termed by them "coral reef areas,"