STRUCTURE OF CORAL REEFS. 123 



instances observed, the layers dip at an angle of six to eight 

 degrees down the beach. This dip is nothing but the slope 

 of the beach itself, and arises from the circumstance that the 

 sands are deposited by the incoming waves, or tides, on such a 

 sloping surface. Tutuila and Upolu, in the Navigator group, 

 and Oahu in the Hawaiian, afford many examples of these 

 beach formations. At certain localities the beach sand-rock has 

 been washed away after it was formed ; and occasionally large 

 masses or slabs have been uplifted by the sea and thrown high 

 up on the beach. 



Deposits of the same kind sometimes include detritus from 

 the hills. Black basaltic pebbles are thus cemented by the 

 white calcareous material, producing a rock of very singular 

 appearance. Near Diamond Hill, on Oahu, is a good locality 

 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 further on. 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 that the rock was not consolidated under water 

 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." 



