hich grow, flourish, and add to the
mass of calcareous matter which already forms the reef. But towards the
lower part of the reef, at a depth of about 120 feet, these creatures
are less active, and fewer of them at work; and at greater depths than
that you find no living coral polype at all; and it may be laid down
as a rule, derived from very extensive observation, that these
reef-building corals cannot live in a greater depth of water than about
120 to 150 feet. I beg you to recollect that fact, because it is one I
shall have to come back to by and by, and to show to what very curious
consequences that rule leads. Well then, coming back to the margin of
the reef, you find that part of it which lies just within the surf to be
coated by a very curious plant, a sort of seaweed, which contains in its
substance a very great deal of carbonate of lime, and looks almost like
rock; this is what is called the nullipore. More towards the land,
we come to the shallow water upon the inside 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