MARINE DEPOSITS 207 



gradually diminish, the large thick-shelled 

 tropical forms disappearing, until in the cold 

 polar waters only one or two dwarfed forms 

 are met with. The percentage of calcareous 

 remains other than those of pelagic foraminifera 

 in the globigerina oozes is also subject to 

 great variation, being on the average about 10 

 or 12, while the remains of siliceous organisms 

 usually make up 1 or 2 per cent., and mineral 

 particles 3 or 4 per cent. The inorganic 

 residue of a globigerina ooze resembles in all 

 respects a red clay and has evidently a similar 

 origin. This type of deposit covers an 

 estimated area of about forty-eight millions 

 of square miles, extending from lat 60° S. 

 in the South Pacific to beyond lat. 70° N. 

 in the Norwegian Sea ; it is specially character- 

 istic of the Atlantic Ocean, where it occurs at 

 greater depths than in the other ocean-basins. 

 7. Pteropod Ooze. — This type may be re- 

 garded as a variety of globigerina ooze, charac- 

 terised by the relatively greater abundance of 

 the shells of pteropods and heteropods fallen 

 from the surface waters. As these pelagic 

 molluscs are to a large extent limited to the 

 warmer waters of the ocean, pteropod oozes 

 are found only in the tropical and subtropical 

 regions, where they occur in less depths than 

 globigerina ooze. This deposit covers an 

 estimated area of about half a million square 



