382 GEOLOGY, 



the average, fall into the sea. But even at this rate it would take some 

 fifty billion years to cover the sea-bottom with a layer one foot in thick- 

 ness. 



Organic constituents of pelagic deposits. — With increasing distance 

 from shores, 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 pelagic deposits are 

 largely made up of shells and skeletons of marine organisms which live 

 in the surface-waters. Pelagic molluscs, foraminifera, and algae secrete 

 shells of lime carbonate, while diatoms and radiolarians secrete shells 

 of siHca. When the organisms die, they sink to the bottom with their 

 shells, and these mineral matters of organic origin are mingled with 

 the volcanic products which are universal over the sea-floor. Pelagic 

 deposits of organic origin are named according to their characteristic 

 constituents. Thus there are pteropod oozes, globigerina 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 diatom shells are abundant, not an 

 ooze made up wholly of diatom shells ; and globigerina ooze is an ooze 

 in which globigerina 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 Hfe does 

 not recognize boundary-Unes. 



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 car- 

 bonate of Ume of the shells of pelagic organisms may constitute 80% 

 or 90% of the deposit. With the same surface conditions, but with 

 increasing depth, the percentage of Ume carbonate decreases, until at 

 2000 fathoms it is less than 60%; at 2400 fathoms, 30%, and at 2600 

 fathoms, 10%. Beyond this depth there are usually no more than 

 traces of carbonate of hme. The data at hand show that the percent- 

 age of lime carbonate falls off below 2200 fathoms more rapidly than at 

 lesser depths. 



* Murray. Scottish Geog. Mag., Vol. XV, p. 511. An excellent summary of deep- 

 sea deposits. 



