154 CORALS AND CORAL ISLANDS. 



for observing the steps in. its formation. Many of the pebbles 

 of the beach are covered with a thin incrustation of carbonate 

 of lime, appearing as if they had been dipped in milk, and 

 others are actually cemented, yet so weakly that the fingers 

 easily break them apart. 



The lime in solution in waters washing over these coral 

 shores is also at times deposited in the cavities or seams of the 

 volcanic rocks ; thus the cavities of a lava or basalt become 

 filled with white calcareous kernels, and the cellular lava is 

 changed into an amygdaloid. In large cavities, or caverns, it 

 often forms stalactites or stalagmitic incrustations. Similar 

 facts are stated by Mr. Darwin as observed on the shores of 

 Ascension ; and many interesting particulars are given respect- 

 ing calcareous incrustations on coasts in his work on Volcanic 

 Islands, some of which are cited beyond. They were observed 

 by the writer upon Madeira, in St. Jago, one of the Cape 

 Verds, as well as among the volcanic islands of the Pacific. 



Jukes speaks of the oolitic character of the beach sand-rock 

 about islets connected with the Australian barrier, and states 

 " that the fact thajt the rock was not consolidated under wa- 

 ter was proved by nests of turtles' eggs being found imbedded 

 in it, these evidently having been deposited by the animal 

 when the sand was above water and still loose and incoherent." 



VII. DRIFT SAND-ROCK. 



Still another kind of beach formation is going on in some 

 regions through the agency of the winds in connection with 

 the sea. It occurs only on the windward side of islands when 

 the reefs are narrow, and proceeds from the drifting of the 

 sand into hillocks or ridges by the winds. 



The drifts resemble ordinary sand-drifts, and are often 



