t24 CORALS AND CORAL LSLANDS. 



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 drifdng of the 

 sand into hillocks or ridges by the winds. 



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 exposed 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 protecting reefs are broad, the 

 author 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 

 friable, which is frequently oolite. The rock consists of thin 

 layers or laminas, which are very distinct, and indicate, gene- 

 rally, every successive drift of sand which pufTs 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 completed 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 the sand-bluffs on the 

 north side of Oahu. Since their formation, this island 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 de- 

 gradation), in which the lamination is finely exposed to view. 

 The layers are but a fraction of an inch ; at one of the 

 hills large slate-like slabs may be obtained ; they have a sanded 

 surface, but are so hard within as to clink under the hammer. 



