212 THE OCEAN 



favourable for the development of radiolarians 

 in great profusion, as in certain parts of the 

 Pacific and Indian Oceans, where radiolarian 

 oozes cover an area estimated at about two 

 millions of square miles ; in the Atlantic this 

 type is quite unknown. 



Murray and Irvine have shown experi- 

 mentally that the remains of siliceous organ- 

 isms are removed in solution, like those of 

 calcareous organisms, and the reason why 

 they are found in greater depths and more 

 abundantly in some deposits than in others 

 depends upon their greater or less abundance 

 in the surface waters. 



Generally speaking, in the deepest regions 

 of the ocean calcium carbonate is either absent 

 or present only in very small percentages, 

 while occasionally there are considerable num- 

 bers of radiolarian and diatom remains (red 

 clays and radiolarian oozes). In the medium 

 depths of the ocean, especially far from land, 

 the dead shells of the pelagic calcareous organ- 

 isms (and in some regions the siliceous organ- 

 isms also) play a predominant role in the 

 formation of the deposits (pteropod ooze, 

 globigerina ooze, and diatom ooze). On the 

 continental slopes and in all enclosed seas there 

 is usually an admixture of pelagic organisms 

 in the deposits with the finer detrital matters 



