1895. RESULTS OF ''CHALLENGER" EXPEDITION. 23 



and sub-tropical regions, far removed from land, the deposits are, 

 except when the depth is very great, chiefly composed of the shells 

 of calcareous organisms, forming the Pteropod and Globigerina 

 oozes. Towards the Antarctic regions, and in the Central and 

 North-west Pacific, the deposits often largely consist of the remains 

 of siliceous organisms, forming the Diatom and Radiolarian oozes. 

 In the very greatest depths of the ocean the calcareous shells of the 

 Pteropod and Globigerina oozes are sometimes entirely removed 

 from the deposits by the solvent action of sea-water, and there is 

 then found the peculiar Red Clay of the oceanic deeps. 



The principal constituents of the Red Clay are siHcate of 

 alumina and the oxides of iron and manganese, which appear to be 

 largely derived from the disintegration of pumice and volcanic dusts. 

 These volcanic materials appear for the most part to have had their 



Fig. I. — Sifting Deposits. 



origin in subaerial volcanic eruptions. Associated with these Red 

 Clays the " Challenger " frequently procured, in a single haul, 

 hundreds of sharks' teeth, some of them of gigantic size and 

 apparently belonging to extinct species, dozens of ear-bones and other 

 bones of whales, large numbers of manganese nodules, zeolitic 

 minerals apparently formed in situ, and magnetic spherules containing 

 native iron, nickel, and cobalt, and believed to be of cosmic origin. 

 The remarkable association of these organic and inorganic materials 

 in the Red Clays seems to indicate an extremely slow rate of accumula- 

 tion, which is the more marked the further these deposits are removed 

 from continental land. 



Chemical changes take place in all the deposits, giving rise to 

 secondary products, such as phosphatic and glauconitic concretions in 



