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 la- 
goons, 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 mater- 
ial 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 dis- 
tance 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 zodphytes; 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 
