THE WORK OF THE OCEAN 329 



Organic constituents of pelagic deposits. With increasing dis- 

 tance 'from shore, and especially with increasing depth of water, 

 terrigenous deposits become less and less abundant, and sediments 

 derived from pelagic life increase in relative importance. Beyond 

 the upper part of the outer slopes of the continental shelves, the 

 deep-sea deposits are made up largely of the secretions of marine 

 organisms which live in the surface-waters. Pelagic molluscs, 

 foraminifera, and algae secrete lime carbonate, while diatoms and 

 radiolarians secrete silica. When the organisms die, they sink to 

 the bottom and their secretions are mingled with the volcanic and 

 other materials which are universal over the sea-floor. 



Pelagic deposits of organic origin are named according to their 

 characteristic constituents. Thus there are pteropod oozes, globi- 

 gerina oozes, diatom oozes, radiolarian oozes, etc. It is not to be 

 understood that these oozes are made up exclusively of the shells 

 which give them their names. Diatom ooze is an ooze in which 

 the secretions of diatoms are abundant, not an ooze made up wholly 

 of these secretions; and globigerina ooze is an ooze in which globi- 

 gerina shells are abundant, though in many cases they do not make 

 up even the bulk of the matter. While samples of these various 

 oozes might be selected which are thoroughly distinct from one 

 another, there are all gradations between them, since pelagic life 

 does not recognize boundary lines. 



It is a significant fact that with increasing depth the proportion 

 of lime carbonate in the ooze decreases. Thus in tropical regions 

 remote from land, where the depths are less than 600 fathoms, 

 the carbonate of lime of the shells of pelagic organisms may con- 

 stitute 80% or 90% of the deposit. With the same surface con- 

 ditions, but with increasing depth, the percentage of lime carbonate 

 decreases, until at 2,000 fathoms it is less than 60%; at 2,400 

 fathoms, 30%, and at 2,600 fathoms, 10%. Beyond this depth 

 there are usually no more than traces of carbonate of lime. The 

 data at hand show that the percentage of lime carbonate falls off 

 below 2,200 fathoms more rapidly than at lesser depths. When 

 the percentage of lime carbonate becomes very low, the calcareous 

 oozes grade off into the red clay with which the sea-floor below 

 2,400 to 2,600 fathoms is covered. 



