STRUCTURE OF CORAL REEFS. 155 



quite extensive. On Ouhu, 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 Hawaian 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 wa- 

 ters, fresh or salf, become cemented into a sand-rock, more or 

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

 thin layers or laminae, 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 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 

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

 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 sand- 

 ed surface, but are so hard within as to clink under the ham- 

 mer. About cavities over the surface, the rock is usually very 

 compact to a depth of half an inch or more, almost horny in 

 texture, owing to the infiltration of lime from the waters often 

 occupying them ; but this is an exceptional variety of the 



