land, or which is perturbed by mud from the same
source, and hence it is that they cease to exist opposite the mouths
of rivers, which damage them in both these ways.

Such is the general distribution of the reef-building corals, but
there are some very interesting and singular circumstances to be
observed in the conformation of the reefs, when we consider them
individually. The reefs, in fact, are of three different kinds; some
of them stretch out from the shore, almost like a prolongation of the
beach, covered only by shallow water, and in the case of an island,
surrounding it like a fringe of no considerable breadth. These are
termed "fringing reefs." Others are separated by a channel which may
attain a width of many miles, and a depth of twenty or thirty fathoms
or more, from the nearest land; and when this land is an island, the
reef surrounds it like a low wall, and the sea between the reef and
the land is, as it were, a moat inside this wall. Such reefs as these
are called "encircling" when they surround an island; and "barrier"
reefs, when they stretch parallel with the coast of a continent.
In both these cases there is ordinary dry land inside the reef, and
separated from it only by a narrower or a wider, a shallower or a
deeper, space of sea, which is called a "lagoon," or "inner passage."
But there is a third kind of reef, of very common occurrence in the
Pacific and Indian Oceans, which goes by the name of an "Atoll." This
is, to all intents and purposes, an encircling reef, without anything
to encircle; or, in other words, without an island in the middle
of its lagoon. The atoll has exactly the appearance of a vast,
irregularly oval, or circular, breakwater, enclosing smooth water in
its midst. The depth of the water in the lagoon rarely exceeds twenty
or thirty fathoms, but, outside the reef, it deepens with great
rapidity to 200 or 300 fathoms. The depth immediately outside the
barrier, or encircling, reefs, may also be very considerable; but, at
the outer edge of a fringing reef, it does not amount usually to more
than twenty or twenty-five fathoms; in other words, from 120 to 150
feet.

Thus, if the water of the ocean could be suddenly drained away, we
should see the atolls rising from the sea-bed like vast truncated
cones, and resembling so many volcanic craters, except that their
sides would be steeper than those of an ordinary volcano. In the case
of the encircling reefs, the cone, with the enclos