GHOLOGICAL CONCLUSIONS. 387 
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 the common mode of origin of odlitéc 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 general slope of the beach sand-rock and odlite, and 
the mixed stratification of the drift sand-rock, are identical re- 
spectively 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 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. 
Il. 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. 263). 
