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, of
course, the shape of the circular land; and then, in the interior of the
circle, there is a pool of water, which is not very deep--probably in
this case not more than eight or nine fathoms--and which forms a strange
and beautiful contrast to the deep blue water outside. This circular
island, or atoll, with a lagoon in the middle, is not a complete circle;
upon one side of it there is a break, exactly like the entrance into a
dock; and, as a matter of course, these circular islets, or atolls, form
most efficient break-waters, for if you can only get inside your ship is
in perfect safety, with admirable anchorage in the interior. If the ship
were lying within a mile of that beach, the water would be one or
two thousand feet deep; therefore, a section of that atoll, with the
soundings as deep as this all round, would give you the notion of a
great cone, cut off at the top, and with a shallow cup in the middle of
it. Now, what a very singular fact this is, that we should have rising
from the bottom of the deep ocean a great pyramid, beside which all
human pyramids sink into the most utter insignificance! These singular
coral limestone structures are very beautiful, especially when crowned
with cocoa-nut tree