FRINGING AND BARRIER REEFS. A5 
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 amygdaloid. In large 
cavities or caverns, it often forms stalactites or stalagmitic incrus- 
tations.* 
Drift sandrock.—Still another kind of beach formation is going on 
in some regions through the agency of the winds in connexion 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 exten- 
sive. 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 exposed to the winds that blow occa- 
sionally from the westward, as well as the regular trades. They also 
occur on Kauai, another of the Hawaiian Islands. But at Upolu, 
(Samoa,) where the protecting 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 sandrock, more or less friable. ‘The 
rock consists of thin layers or lamine, which are very distinct, and 
indicate, generally, every successive drift of sand which puffs of wind 
had added in the course of its formation: and where a heavier gale 
had blown off the top of a drift, and new accumulations again com- 
pleted it, the whole history is distinctly displayed in the rock. 
Several catastrophes of this kind may be made out from the character 
of the lamination in one of the sand-bluffs on the north side of Oahu. 
This island, since their formation, has undergone an elevation of 
twenty-five or thirty feet; these hills, once on the shores, are now 
seventy feet above the level of the sea, and they face the water with a 
bluff front (due to degradation), in which the lamination is finely 
exposed to view. The structure is best seen in a transverse section, 
presented on the west side. The layers are but a fraction of an inch 
* 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 us upon Madeira, in St. Jago, one of 
the Cape Verds, as well as among the basaltic islands of the Pacific. 
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