Corals and coral islands.

232

CORALS AND CORAL ISLANDS.

source of this kind of material in the self-triturating sands of
the reefs acted upon by the moving waters. On the seaward
side of coral islands, and on the shores of the larger lagoons, where the surface rises into waves of much magnitude,
the finer portions are carried off, and the coarser sand remains
alone to form the beaches. This making of coral sand and
mud is just like that of any other kind of sand or mud. It
takes place on all shores exposed to the waves, coral or not
coral, and in every case the gentler the prevailing movement
of the water, the finer the material on the shore. In the
smaller lagoons, where the water is only rippled by the winds,
or roughened for short intervals, the trituration is of the
gentlest kind possible, and, moreover, the finely pulverized
material remains as part of the shores. Thus the fine material of the mud must be constantly forming on all the shores,
for the sands are perpetually wearing themselves out; but the
particles of the fine mud, which is washed out from the beach
sands, accumulates only in the more quiet waters some distance outside of the reef, and within the lagoons and channels,
where it settles. This corresponds exactly with the facts; and
every small lake or region of quiet waters over our continent,
illustrates the same point.
Mr. Darwin, in discussing the origin of the finer calcareous
mud, (op. cit., p. 14), supposes that it is derived in part from
fishes and Holothurians; and other authors have thrown out
the same suggestion. He cites as a fact, on the authority of
Mr. Liesk, that certain fish browse on the living zoiphytes; and
from Mr. Allan, of Forres, he learned also that Holothurians
subsisted on them. The statement about the Holothurians
has been set aside by observation. Small fish swarm about
the branching clumps, and when disturbed, seek shelter at once
among the branches, where they are safe from pursuit. The