On coral reefs and islands . nd 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 isdistinctly displayed in the rock. Several catastrophes of thiskind may be made out from the character of the lamination inthe sand-bluffs on the north side of Oahu. Since their forma-tion, this island has undergone an elevation of twenty-five orthirty feet; these hills, once on the shores, are now seventyfeet above the level of the sea, and they face the water with abluff front (due to degradation), i
On coral reefs and islands . nd 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 isdistinctly displayed in the rock. Several catastrophes of thiskind may be made out from the character of the lamination inthe sand-bluffs on the north side of Oahu. Since their forma-tion, this island has undergone an elevation of twenty-five orthirty feet; these hills, once on the shores, are now seventyfeet above the level of the sea, and they face the water with abluff front (due to degradation), in which the lamination is finelyexposed to view. The structure is best seen in a transverse sec-tion, presented on the west side. The layers are but a fractionof an inch thick: at one of the hills large slate-like slabs maybe obtained; they have a sanded surface, but are so hard withinas to clink under the hammer. A particular description of thesebluffs is given in the authors remarks on the geology of the Ha-waiian BLUFFS OF CORAL SAND-ROCK, NORTH SHORE OF OAHU. One of the most interesting facts, observed in connection withthese drift hills, is the absence of shells, and even of fragmentsof shells or corals, sufficiently large to be referred to either ofthese sources. The material is a fine sand, without organic re-mains, although situated on shores off which, within a hundredyards, there are shells and corals innumerable. STRUCTURE OF CORAL REEFS. 19 C. Thickness of reefs. We have considered in the preceding pages the peculiarities ofform and structure characterizing the reef formations borderingislands and continents, and their influence upon the enclosed we raise one of these coral-bound islands from the waves,we should find that the reefs stand upon the submarine slopes,like massy structures of artificial masonry ; some forming a broadflat platform or shelf ranging around the land, and others encirclingit like vast ramparts, perhaps a hundred miles or
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