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 1,100 miles off the east coast of Australia. Multitudes
of the island 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, 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