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 the Pacific. You must fancy yourself
in the middle of the great ocean, and you will perceive that there is
an almost circular island, with a low beach, which is formed entirely
of coral sand; growing upon that beach you have vegetation, which takes,