side of the reef, which has
a particular name, derived from the Spanish or the Portuguese--it is
called a "lagoon," or lake. In this lagoon there is comparatively little
living coral; the bottom of it is formed of coral mud. If we pounded
this coral in water, it would be converted into calcareous mud, and the
waves during storms do for the coral skeletons exactly what we might do
for this coral in a mortar; the waves tear off great fragments and
crush them with prodigious force, until they are ground into the merest
powder, and that powder is washed into the interior of the lagoon, and
forms a muddy coating at the bottom. Beside that there are a great many
animals that prey upon the coral--fishes, worms, and creatures of that
kind, and all these, by their digestive processes, reduce the coral to
the same state, and contribute a very important element to this fine
mud. The living coral found in the lagoon, is not the reef building
coral; it does not give rise to the same massive skeletons. As you go
in a boat over these shallow pools, you see these beautiful things,
coloured red, blue, green, and all colours, building their houses;
but these are mere tenements, and not to be compared in magnitude
and importance to the masses which are built by the reef-builders
themselves. Now such a structure as this is what is termed a "fringing
reef." You meet with fringing reefs of this kind not only in the
Mauritius, but in a number of other parts of the world. If these were
the only reefs to be seen anywhere, the problem of the formation of
coral reefs would never have been a difficult one. Nothing can be
easier than to understand how there must have been a time when the coral
polypes came and settled on the shores of this island, everywhere within
the 20 to 25 fathom line, and how, having perched there, they gradually
grew until they built up the reef.

But these are by no means the only sort of coral reefs in the world; on
the contrary, there are very large areas, not only of the Indian ocean,
but of the Pacific, in which many many thousands of square miles
are covered either with a peculiar kind of reef, which is called the
"encircling reef," or by a still more curious reef which goes by the
name of the "atoll." There is a very good picture, which Professor
Roscoe has been kind enough to prepare for me, of one of these atolls,
which will enable you to form a notion of it as a landscape. You have in
the foreground the waters of t