, and frequently, though not
always, descends abruptly into great depths. In many cases in the
Pacific Ocean a barrier reef surrounds one or more island peaks, and the
strips of land on the edge of the reef may encircle the peaks with a
nearly complete ring. An _atoll_ is a ring-shaped reef, either awash at
low tide or surmounted by several islets, or more rarely by a complete
strip of dry land surrounding a central lagoon. The outer wall of an
atoll generally descends with a very steep but irregular slope to a
depth of 500 fathoms or more, but the lagoon is seldom more than 20
fathoms deep, and may be much less. Frequently, especially to the
leeward side of an atoll, there may be one or more navigable passages
leading from the lagoon to the open sea.

Though corals flourish everywhere under suitable conditions in tropical
seas, coral reefs and atolls are by no means universal in the torrid
zone. The Atlantic Ocean is remarkably free from coral formations,
though there are numerous reefs in the West Indian islands, off the
south coast of Florida, and on the coast of Brazil. The Bermudas also
are coral formations, their high land being formed by sand accumulated
by the wind and cemented into rock, and are remarkable for being the
farthest removed from the equator of any recent reefs, being situated in
32 deg. N. lat. In the Pacific Ocean there is a vast area thickly dotted
with coral formations, extending from 5 deg. N. lat. to 25 deg. S. lat.,
and from 130 deg. E. long, to 145 deg. W. long. There are also extensive
reefs in the westernmost islands of the Hawaiian group in about 25 deg.
N. lat. In the Indian Ocean, the Laccadive and Maldive islands are large
groups of atolls off the west and south-west of India. Still farther
south is the Chagos group of atolls, and there are numerous reefs off
the north coast of Madagascar, at Mauritius, Bourbon and the Seychelles.
The Cocos-Keeling Islands, in 12 deg. S. lat. and 96 deg. E. long., are
typical atolls in the eastern part of the Indian Ocean.

[Illustration: Diagram showing the formation of an atoll during
subsidence. (After C. Darwin.) The lower part of the figure represents
a barrier reef surrounding a central peak.

A, A, outer edges of the barrier reef at the sea-level; the coco-nut
trees indicate dry land formed on the edges of the reef.

L, L, lagoon channel.

A', A', outer edges of the atoll formed by upgrowth of the coral
during the subsidence