350 CORALS AND CORAL ISLANDS. 



of very large areas of this compact white limestone, and also 

 for others of impure or argillaceous limestones. 



Besides the kinds of coral rocks above mentioned, there are 

 also the Beach and Drift Sand-rocks, which are accumulated 

 and consolidated above low-tide level. These formations illus- 

 trate 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 re- 

 spectively with those of l3each 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 fur- 

 nishes 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 156.) The reefs are, 

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

 markable 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. 258). 



