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 "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 two hundred or three hundred 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 one hundred and
twenty to one hundred and fifty feet.
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