MARINE DEPOSITS 197 



and deposits of pteropod or globigerina ooze 

 may contain over 80 per cent, of calcium 

 carbonate, due to the presence of these pelagic 

 shells. 



Again, the siliceous frustules of diatoms, 

 or the siliceous spicules and skeletons of radio- 

 laria, may predominate in the deposits far 

 from land. These have fallen to the bottom 

 after the death of the organisms, just as in 

 the case of the pteropods and pelagic foramini- 

 fera. But in marine deposits the siliceous 

 spicules of sponges which lived on the bottom 

 can also be detected. 



The foregoing are then the principal con- 

 stituents of marine deposits, but there are 

 others which are not so abundant, such as 

 small spherules of extra-terrestrial origin, and 

 secondary products which have been formed 

 in situ in the deposits, such as manganese-iron 

 nodules, phosphatic, barytic and calcareous 

 nodules, glauconite, and zeolitic crystals. 



Some things that we should expect to find 

 are extremely rare. The ordinary bones of 

 fishes are seldom observed in the deposits, with 

 the exception of teeth and otoliths, and the 

 teeth of sharks, the earbones of whales and the 

 dense beaks of ziphioid whales have been 

 dredged in considerable numbers in some 

 areas, but these are usually much corroded ; 

 the more areolar bones of whales are repre- 



