Corals and coral islands.

STRUCTURE OF CORAL REEFS.

159

IX. A GOOD WORD FOR CORAL REEFS.
All coral-bound coasts, and especially those of islands in mid
ocean, derive great benefit from their reefs. The wide coral banks
and the enclosed channels greatly enlarge the liinits tributary
to the lands they encircle. Besides being barriers against the
ocean, they are dykes to detain the detritus of the hills.
They stop the waters of the streams, and cause it to drop the
silt they were bearing off, and thus secure its addition to the land.
They prevent, therefore, the waste which is constantly going on
about islands without such barriers; for the ocean not only encroaches upon the unprotected shores of small islands, but carries off much of whatever the streams empty into it. The delta of Rewa, on Viti Lebu, resulting from the detritus accumulations of a large river, covers nearly sixty square miles. This
is an extreme case in the Pacific, as few islands are so large,
and consequently rivers of such magnitude are not common.
But there is rarely a coral-girt island which has not at least
some narrow plains from this source; and upon them the villages of the natives are usually situated. Around Tahiti these
plains are from half a mile to two or three miles in width, and
the cocoa-nut and bread fruit groves are mostly confined to
them.
The reefs also provide extensive fishing-grounds for the natives, and afford abundant fish, their main reliance in the way
of animal food. They also supply large interior waters for
practice in navigation and for safe communication between distant settlements. And the effect is evident in the spirit of
maritime enterprise which characterizes the islanders; for
these circumstances have favored the construction of large sailcanoes in which they venture beyond their own land, and often
undertake voyages hundreds of miles in length. Communica