DEEP-SEA DEPOSITS 199 



they sink to the bottom and their secretions are mingled with the 

 volcanic and other mate-rials 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- 

 gcrina oozes, diatom oozes, radiolarian oozes, etc. Diatom ooze is 

 an ooze in which the secretions of diatoms are abundant, and 

 globigerina ooze is an ooze in which globigerina shells are abundant, 

 though in many cases the diatom and globigerina shells, respectively, 

 do not make up the bulk of the ooze. Between the various sorts of 

 oozes there are all gradations, 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 depth is less than 600 fathoms, the 

 carbonate of lime of the shells of pelagic organisms may constitute 

 80% or 90% of a deposit. With the same surface conditions, 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. Where 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. 



Chemical deposits. The chemical deposits of the deep sea are 

 chiefly the alteration products of sediments which reach the sea- 

 bottom by mechanical means. All sediment deposited in the sea 

 undergoes more or less chemical change, but it is only when the 

 change is very considerable that the product is referred to this class. 

 Where sedimentation is rapid and the sediment coarse, the chemical 

 change is relatively slight; but where the sedimentation is slow 

 and the sediment fine, the chemical change is relatively great; for 

 both the longer exposure to the sea-water and the greater propor- 

 tion of surface exposed to attack favor change. The red day 

 already referred to belongs to this class of deposits. It contains 

 much volcanic debris, 1 various concretions, bones of mammals, 

 zeolitic crystals, and extra-terrestrial spherules, and doubtless the 



1 Murray, Challenger Report on Deep Sea Deposits, p. 337 et seq., and Buchan- 

 an, Proc. Roy. Soc. EoUn., Vol. XVIII, pp. 17-39. 



