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,"
or regions in which coral reefs are found. There is a very notable
example of a simple coral reef about the island of Mauritius, which I
dare say you all know, lies in the middle of the Indian Ocean. It is a
very considerable and beautiful island, and is surrounded on all sides
by a mass of coral, which has been formed in the way I have described;
so that if you could get upon the top of one of the peaks of the island,
and look down upon the Indian Ocean, you would see that the beach round
the Island was continued outward by a kind of shallow terrace, which
is covered by the sea, and where the sea is quite shallow; and at a
distance varying from three-quarters of a mile to a mile and a half from
the proper beach, you would see a line of foam or surf which looks most
beautiful in contrast with the bright green wat