92 THE OCEAN FLOOR 



If we assume the first of the three factors mentioned 

 above as dominant, then during the Tertiary Age — 

 when the bottom water was much warmer than at 

 present and no driving power for the bottom currents 

 in the shape of melting ice existed — no extensive layers 

 of red clay of Pre-Quaternary age should have been 

 formed. The "Albatross" Expedition negatived this 

 conclusion by raising long red clay cores both in the 

 Atlantic and the Pacific, to the lower layers of which 

 a Tertiary age must be ascribed. It seems reasonable, 

 therefore, to assume that magmatic volatiles set free by 

 submarine volcanic activity, together with the intense 

 local bottom currents which it must have produced, 

 were the main agents for converting calcareous ooze 

 into red clay in the Tertiary Age. 



Near the equator especially, cores were raised in 

 which the lime content showed abrupt variations, so 

 that red clay alternated with calcareous ooze. The 

 causes of this stratification are at present being investi- 

 gated. 



The other biogenetic component of primary im- 

 portance is silica, in the shape of shells or skeletons 

 from siliceous algae (diatoms) or protozoans (radio- 

 larians). The former, which are in general cold-water 

 organisms, are encountered chiefly in polar and sub- 

 polar regions. The latter, on the contrary, are found at 

 low latitudes. When the content of organic silica ex- 

 ceeds 20*; t we speak of diatom ooze or radiolarian 

 ooze. Like the calcareous remains in the bottom de- 



