Coral and Coral Reefs | Page 7

of the reef, you find that the depth of the water is not more
than from 20 to 25 fathoms--that is about 120 to 150 feet. Outside that
point you come to the natural sea bottom; but all inside that depth is
coral, built up from the bottom by the accumulation of the skeletons of
innumerable generations of coral polypes. So that you see the coral
forms a very considerable rampart round the island. What the exact
circumference may be I do not remember, but it cannot be less than 100
miles, and the outward height of this wall of coral rock nowhere
amounts to less than about 100 or 150 feet.

When the outward face of the reef is examined, you find that the upper
edge, which is exposed to the wash of the sea, and all the seaward face,
is covered with those living plant-like flowers which I have described
to you. They are the coral polypes which 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 nulli pore. 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 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