GEOLOGICAL CONCLUSIONS. 301 



and consolidated above low-tide level. These formations 

 illustrate one common mode of origin of oolitic limestones. 

 They also afford numerous examples of the formation of coarse 

 and fine conglomerates consisting of beach pebbles — these 

 pebbles being either worn corals, or shells, or sometimes of 

 other kinds, if other rocks are at hand. 



The uniform slope of the beach sand-rock, and oolite, and 

 the mixed stratification of the drift sand-rock, are identical 

 respectively with those of beach and drift-sand deposits in 

 other regions. 



II. BEDS OF LIMESTONE WITH LIVING MARGINS. 



The coral reef as it lies at the water's level is in fact a bed 

 of limestone with living margins ; and the living part furnishes 

 material for its horizontal extension outward, and also, if a 

 slow subsidence is in progress, for its increase upward. It 

 illustrates an ordinary mode of formation of coral, or of shell, 

 limestone, whatever the age. 



III. MAKING OF THICK STRATA OF LIMESTONE. 



The coral reef-rock has been shown to have in some cases 

 a thickness of at least 2,000 feet (page 126). The reefs are, 

 therefore, examples of great limestone strata, nearly as remark- 

 able in this respect as the largest of ancient times. 



IV. SUBSIDENCE ESSENTIAL TO THE MAKING OF THICK STRATA. 



The coral island reef-rock has been shown to depend for its 

 thickness on a slowly progressing subsidence (p. 221). This 

 is the only method by which any thick stratum of limestone 

 could be made out of a single set of species, for all such 

 species have a narrow range in depth ; and the only way, from 

 any succession of species, if those species are alike in range 

 of depth. 



In the case of existing coral reefs, there is yet no evidence 

 that the species of the lower beds differ from those of the top. 



