154 CORALS AND CORAL ISLANDS. 
for observing the steps in its formation. Many of the pebbles 
ot 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 odlitic character of the beach sand-rock 
about islets connected with the Australian barrier, and states 
“that the fact that 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.” 
Vil. 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 

