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 water in the inside, and
the deep blue of the sea beyond. That line of surf indicates the point
at which the waters of the ocean are breaking upon the coral reef which
surrounds the island. You see it sweep round the island upon all
sides, except where a river may chance to come down, and that always
makes a gap in the shore.

There are two or three points which I wish to bring clearly before your
notice about such a reef as this. In the first place, you perceive it
forms a kind of fringe round the island, and is therefore called a
"fringing reef." In the next place, if you go out in a boat, and take
soundings at the edge 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