ABYSSAL DEPOSITS 675 



deposits. The former group may be termed the holopelagic group, 

 since it comprises organisms permanently at home in the pelagic 

 district, while the latter may be termed the meropelagic group, com- 

 prising types at home in the pelagic district during part of their 

 lives only. 



Foraminifera, ostracods and pteropod shells are calcareous, and 

 these form the chief source of the lime deposits of the deep sea. 

 Foraminiferal oozes of the modern sea generally abound in the 

 shells of Globigerina, hence the term Globigerina ooze is ap- 

 plied to them. It should be emphasized, however, that the For- 

 aminifera of this ooze are pelagic species, i. e., types which float in 

 the pelagic district of the open ocean. This point is often over- 

 looked in interpreting former foraminiferal oozes as deep-sea de- 

 posits. Thus it has been shown repeatedly that the Foraminifera 

 of the chalk are largely of littoral types with only such pelagic ad- 

 ditions as might be expected from the fact that the pelagic district 

 really overlaps the littoral. Nevertheless the chalk is commonly 

 compared with the modern Globigerina ooze, and referred to abys- 

 sal deposits, whereas the Foraminifera composing it point rather to 

 a littoral origin. 



The great purity of deep-sea oozes of pelagic origin is due to 

 the fact that the amount of terrigenous material settling here is 

 relatively slight. Thus in tropical seas, in depths of 600 fathoms 

 or less, pelagic shells of carbonate of lime often constitute 80 

 per cent, to 90 per cent, of the deposit. With increasing depth, 

 however, the percentage of carbonate of lime decreases, though the 

 surface conditions are the same. Thus at 2,000 fathoms the lime 

 is less than 60 per cent., at 2,400 fathoms 30 per cent, and at 2,600 

 fathoms 10 per cent. (Chamberlin and Salisbury-i6: j5'i'), while 

 below this lime is generally absent. The most rapid falling off of 

 the percentage of lime carbonate is below 2.200 fathoms, while be- 

 tween 2,400 and 2,600 fathoms the floor of the ocean is covered 

 with red clay. The explanation generally given for this decrease 

 in the percentage of carbonate of lime is the greater power of solu- 

 tion of the deeper waters, owing either to the greaf pressure under 

 which it exists or to the abundance of CO2 in it derived from emana- 

 tions from the sea floor, or to both causes. (For other factors in- 

 fluencing this, see Philippi-69.) The red clay of the deeps has 

 been regarded as the insoluble residue left on solution of these 

 shells, a not unlikely source for at least a part of this deposit. 



Radiolaria and diatom shells are siliceous, and often constitute 

 extensive deposits in the modern oceans. Diatoms are, however, 

 not confined to salt water, but occur in fresh water as well. Ex- 



