18 



ON CORAL REEFS AND ISLANDS. 



able. The rock consists of thin layers or laminas, which are 

 very distinct, and indicate, generally, every successive drift of 

 sand which puffs of wind had added in the course of its forma- 

 tion : 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 forma- 

 tion, 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 degradation), in which the lamination is finely 

 exposed to view. The structure is best seen in a transverse sec- 

 tion, presented on the west side. The layers are but a fraction 

 of an inch thick : 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. A particular description of these 

 bluffs is given in the author's remarks on the geology of the Ha- 

 waiian islands. 



BLUFFS OF CORAL SAND-ROCK, NORTH SHORE OF OAHU. 



One of the most interesting facts, observed in connection with 

 these drift hills, is the absence of shells, and even of fragments 

 of shells or corals, sufficiently large to be referred to either of 

 these sources. The material is a fine sand, without organic re- 

 mains, although situated on shores off which, within a hundred 

 yards, there are shells and corals innumerable. 



