ck ridges, the seaman gives the
name of "reefs."

Such coral reefs cover many thousand square miles in the Pacific and in
the Indian Oceans. There is one reef, or rather great series of reefs,
called the Barrier Reef, which stretches, almost continuously, for more
than eleven hundred miles off the east coast of Australia. Multitudes
of the islands in the Pacific are either reefs themselves, or are
surrounded by reefs. The Red Sea is in many parts almost a maze of such
reefs, and they abound no less in the West Indies, along the coast of
Florida, and even as far north as the Bahama Islands. But it is a very
remarkable circumstance that, within the area of what we may call the
"coral zone," there are no coral reefs upon the west coast of America,
nor upon the west coast of Africa; and it is a general fact that the
reefs are interrupted, or absent, opposite the mouths of great rivers.
The causes of this apparent caprice in the distribution of coral reefs
are not far to seek. The polypes which fabricate them require for their
vigorous growth a temperature which must not fall below 68 degrees
Fahrenheit all the year round, and this temperature is only to be
found within the distance on each side of the equator which has been
mentioned, or thereabouts. But even within the coral zone this degree of
warmth is not everywhere to be had. On the west coast of America, and on
the corresponding coast of Africa, the currents of cold water from the
icy regions which surround the South Pole set northward, and it appears
to be due to their cooling influence that the sea in these regions is
free from the reef builders. Again, the coral polypes cannot live in
water which is rendered brackish by floods from the 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