not have coral three or four times as big; and there are certain
creatures of this kind that do fabricate very large masses, or half
spheres several feet in diameter. Thus the activity of these animals
in separating carbonate of lime from the sea and building it up into
definite shapes is very considerable indeed.

Now I think I have said sufficient--as much as I can without taking you
into technical details, of the general nature of these creatures which
form coral. The animals which form coral are scattered over the seas of
all countries in the world. The red coral is comparatively limited, but
the polypes which form the white coral are widely scattered. There
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 skele