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 mollusks and echini, with 

 their broken-orT 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 lift 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, 



