STRUCTURE OF CORAL REEFS. 125 



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 Ume from the waters often 

 occupying them ; but this is an exceptional variety of the 

 rock. A particular description of these bluffs is given in the 

 author's report on the geology of the Hawaiian Islands. 



One of the most interesting facts observed in connection 

 with these drift hills, is the absence of shells, and even of fra^-- 

 ments of shells or corals sufficiently large to be referred to 

 either of these sources. The material is sand, without organic 

 remains, although situated on shores off which, within a hun- 

 dred yards, there are shells and corals innumerable. The grind- 

 ing action of the waves and winds reduces all the coral frag- 

 ments and shells, by mutual trituration, to a fine beach sand. 



Oolitic beds appear to be confined to the superficial forma- 

 tions of a reef, that is, to the beach and wind-drift accumulations. 

 No example has come under the notice of the author of oolite 

 constituting the foundation rock of a reef or island. It is possi- 

 ble that such beds might in some cases be the basement rocks to 

 a considerable depth below ; for a reef-island might subside so 

 much more slowly than coral formations grow and accumulate, 

 that a succession of beach-made beds would be produced even 

 to a great thickness. Yet the probability is that the subsi- 

 dence would sink the surface beneath the water, and put an 

 end to beach and wind-drift work. The beach slope of 6° to 

 8" is an almost constant mark of beach-made beds. 



VITI. THICKNESS OF REEFS. 



We have considered in the preceding pages the peculiari- 

 ties of form and structure characterising the reef formations 

 bordering islands and continents, and their influence upon the 

 inclosed land. Could 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 broad flat platform or shelf ranging around 



