192 FOUNDERS OF OCEANOGRAPHY 



4. Radiolarian Ooze. — This deposit covers about two 

 millions of square miles at the greatest depths in a few 

 isolated areas in the tropical Pacific and Indian Ocean. 

 It does not occur in the Atlantic, nor in the great Southern 

 Ocean. Its range is from about 2,500 to 6,000 fathoms, 

 but is determined apparently not so much by the depth 

 as by the presence of enormous quantities of Radiolaria 

 (with siUceous shells) in the surface waters of these localities. 

 The foundation of the deposit is Red Clay, of which it 

 may be considered to be a variety in which the siUceous 

 shells of Radiolaria are so abundant as to give a character- 

 istic appearance under the microscope, and on analysis. 

 The other mineral constituents, apart from the silica, which 

 forms about 25 per cent, of the ooze, are those found in 

 Red Clay. Radiolaria shells are found in smaller quantities 

 in Globigerina ooze and in Red Clay and other deposits, in 

 fact, wherever there are Radiolaria living in the surface 

 waters above, but in these cases the minute and delicate 

 siliceous shells are masked by the greater quantity and 

 larger size and opacity of the Foraminifera and other 

 organisms. It is only when, at depths over 2,500 fathoms, 

 the calcareous shells have been dissolved away by the car- 

 bonic acid in the sea-water that the delicate Radiolaria 

 shells, and some Diatom frustules, become conspicuous. 

 Even sihceous shells, however, have been shown by Murray 

 and Irvine to be dissolved to some extent in sea-water, and 

 therefore it is only when the Radiolaria are present in great 

 abundance on the surface, as in the tropical Pacific and 

 Indian Oceans, that what is left of their remains are sufficient 

 to form a Radiolarian ooze at the bottom. 



5. Diatom Ooze. — This is also, like Radiolarian ooze, a 

 siliceous deposit, and is formed of the frustules or valves 

 of Diatoms where these microscopic plants are present in 

 enormous abundance in cold surface circumpolar waters. 

 It occupies about 10 milhons of square miles at depths of 

 600 to 2,000 fathoms, and is characteristic of the Antarctic 



