SUBMARINE DEPOSITS. 



287 



■ Rock from Fourtales Plateau. 



time approaching the reef. The plateau begins at a depth of 

 about 90 fathoms, and ends at about 300. 



The bottom, as described by Pourtales, is rocky, rather rough, 

 and consists of a recent Umestone (Fig. 192), continually though 

 slowly increasing 

 from the accumula- 

 tion of the calcare- 

 ous debris of the 

 numerous small 

 corals, e c h i n o - 

 derms, and m o 1 - / 

 lusks living on its 

 surface. These 

 debris are consoli- 

 dated by tubes of 

 serpulae ; the inter- 

 stices are filled up ^■^s-^.Q^.- 

 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 the 

 straits. The bottom here forms a great bed of foraminifera, 

 and especially of globigerinse, which so extensively covers 

 oceanic basins. 



If uncovered, the Pourtales 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 pelagic forms as 

 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 singularly 

 barren ; but beyond this, we find a region remarkably rich in 

 animal forms. The fauna found on the Pourtales 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 instance, recent dolomite 

 ment to determine the specific gravity of and recent limestones with the specific 

 rocks taken from deep water, and to gravity of similar rocks. 



