STRUCTURE OF CORAL REEFS. 17 



In all instances observed, these calcareous sand-rocks or con- 

 glomerates form a number of parallel layers along the coast, which 

 dip regularly at an angle of five to eight degrees towards the wa- 

 ter. The layers are from a few inches to a foot in thickness. 

 They appear as if they had been tilted by some force from below, 

 and are seen to outcrop successively, on receding from the water. 

 Tutuila and Upolu in the Navigator Group, and Oahu in the Ha- 

 waiian, afforded us many examples of these beach formations. 

 They seldom rise more than a few inches above high tide. At 

 certain localities they appear to have been washed away after they 

 were formed ; and occasionally large masses or slabs have been 

 uplifted by the sea, and thrown back on the beach. 



Deposits of the same kind sometimes included 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 cov- 

 ered 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 basaltic 

 rocks ; the cavities of the lava or basalt become filled with white 

 calcareous kernels, and the cellular lava is changed into an amyg- 

 daloid. In large cavities or caverns, it often forms stalactites or 

 stalagmitic incrustations.* 



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 drift 

 sands. 



The drifts resemble ordinary sand-drifts, and are often quite 

 extensive. On Oahu, they occur at intervals around the eastern 

 shores, from the northern cape, to Diamond Point which forms 

 the south cape of the island, — the part exposed to the trades ; 

 and they are in some places twenty to forty feet in height. They 

 are most remarkable on the north cape, a prominent point ex- 

 posed to the winds that blow occasionally from the westward, as 

 well as to the regular trades. They also occur on Kauai, another 

 of the Hawaiian Islands. But at Upolu, (Samoa,) where the pro 

 tecting reefs are broad, I met with no instance worthy of mention. 



These sand-banks, through the agency of infiltrating waters, 

 fresh or salt, become cemented into a sand-rock, more or less fri- 



* Similar facts are stated by Mr. Darwin as observed on the shores of Ascension, 

 and many interesting particulars are given respecting calcareous incrustations on 

 coasts. — See Vole. Islands, p. 49. They were observed by the writer upon Madeira, 

 in St. Jago, one of the Cape Verds, as well as among the basaltic islands of the Paciiic. 



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