170 
CORAL ANIMALCULES. 
of a submarine crater, contain in the centre a com- 
paratively shallow lagoon or lake, where there is 
still water, and on the borders of which the smaller 
and more delicate kind of zoophytes find a tranquil 
abode, while the hardier species live on the exte- 
rior margin of the islet, where a great surf usually 
^breaks. When the reef is of such a height that it 
remains almost dry at low water, the corals leave 
off building. 
A continuous mass of solid stone is now seen, 
composed of the shells of moUusks and echini, with 
their broken- off prickles and fragments of corals 
united by calcareous sand produced by the pulveri- 
zation of shells. Fragments of coral limestone are 
thrown up by the waves, until the ridge becomes 
so high that it is covered only during some seasons 
of the year by the high tides. The heat of the sun 
often penetrates the mass of stone when it is dry, 
so that it splits in many places. The force of the 
waves is thereby enabled to separate and hft blocks 
of coral frequently six feet long and three or four 
in thickness, and throw them upon the reef. After- 
ward the calcareous sand lies undisturbed, and of- 
fers to the seeds of trees and plants cast upon it 
by the waves, a soil upon which they rapidly grow 
to overshadow its dazzling white surface. Trunks 
and branches of trees, which are carried by the riv- 
ers from other countries and islands, find here, at 
length, a resting-place after their long wanderings ; 
with these come some small animals, such as liz- 
ards and insects, for the first inhabitants. Even 
before the trees form a wood, the sea-birds nestle 
here ; strayed land-birds take refuge in the bushes ; 
and, at a much later period, when the work has 
been long since completed, man appears and builds 
his hut on the fruitful soil.* 
Most coral reefs contain a greater or less pro- 
* Lyell and Kotzebue. 
