SUBMARINE DEPOSITS. 287 
time approaching the reef. The plateau begins at a depth of 
about 90 fathoms, and ends at about 300. 
The bottom, as described by Pourtalés, is rocky, rather rough, 
and consists of a recent limestone (Fig. 192), continually though 
slowly increasing 
from the accumula- 
tion of the calcare- 
ous débris of the 
numerous small 
corals, echino- 
derms, and mol- 
lusks living on its 
surface. These 
débris are consoli- 
dated by tubes of 
serpulæ ; the inter- 
stices are filled up 
by foraminifera, and further smoothed over by nullipores. The 
region of this recent limestone! ceases at a depth varying from 
250 to 350 fathoms, and beyond it comes the trough of one 
straits. The bottom here forms a great bed of for vibe ет: 
апа especially of globigerine, which so extensively covers 
oceanic basins. 
If uncovered, the Pourtalés Plateau would be found to be built 
up of beds of limestone filled with fossils, the remains of the 
animals now living on its bottom, with such pelagie forms as 
Fig. 192. — Rock from Pourtalés Plateau. 
have sunk after death like the remains of fishes and of pelagic 
mollusks, or which, like the bones of manatee, may have 
been brought by currents from the littoral regions. The rich 
fauna of the reef extends but slightly seaward, and until we 
reach about the hundred-fathom line the coast shelf is singul: arly 
barren; but beyond this, we find a region remarkably rich in 
animal forms. The fauna found on the Pourtalés Plateau is 
undoubtedly due to the action of the Gulf Stream, which sup- 
plies the animals living upon it with an abundance of food, and, 
1 It would be an interesting experi- compare, for instanee, recent dolomite 
ment to determine the specific gravity of and reeent limestones with the specific 
rocks taken from deep water, and to gravity of similar rocks. 
