IV DEPTHS AND DEPOSITS OF THE OCEAN 171 



As to the relative rate of accumulation of the different types 

 of deposits, it may be assumed that the terrigenous deposits 

 accumulate at a much more rapid rate than the pelagic deposits. 

 Of the terrigenous deposits, the Blue muds situated near the 

 mouths of large rivers may be supposed to accumulate at a 

 relatively very rapid rate, for the various constituents of the mud 

 show little trace of alteration, while the rate of deposition in 

 the case of Green muds and sands must be much slower, since the 

 mineral particles are generally profoundly altered, and there is 

 an extensive formation of secondary products, like glauconite 

 and phosphate of lime ; Coral muds and sands appear to accumu- 

 late rapidly under certain conditions, and the same may be said 

 of Volcanic muds and sands in the neighbourhood of active 

 volcanoes, where the volcanic minerals are fresh and unaltered, 

 but most of the deep-sea volcanic deposits far from land appear 

 to accumulate at a relatively slow rate, for the volcanic particles 

 show abundant traces of alteration accompanied by the deposi- 

 tion of manganese peroxide. 



Of the pelagic deposits, the Globigerina and Pteropod oozes 

 of tropical regions probably accumulate the most rapidly, from 

 the greater variety of tropical pelagic species of foraminifera 

 and molluscs, and the larger and more massive shells secreted in 

 tropical as compared with extra-tropical regions. Diatom ooze 

 appears to accumulate at a more rapid rate than Radiolarian 

 ooze, since in addition to the siliceous remains it usually 

 contains a considerable admixture of calcareous remains, but 

 from all points of view it seems reasonable to suppose that the 

 minimum rate of deposition of materials on the ocean-floor is 

 reached in those characteristic Red clay areas farthest removed 

 from continental land and in very deep water. The greater 

 abundance of cosmic spherules, sharks' teeth, and ear-bones of 

 whales, some of them belonging to extinct species, in the Red 

 clays than in any other type of deposit, is ascribed to the fact 

 that few other substances there fall to the bottom to cover them 

 up. The state of profound alteration of the volcanic materials 

 in the Red clay, accompanied by the secondary formation of 

 clay, manganese nodules, and zeolitic crystals, is ascribed to the 

 fact that these materials have lain for a long time exposed to 

 the solvent action of sea-water. The presence of radio-active 

 substances in this deposit, in much larger quantity than in other 

 deposits, apparently also points to a very slow rate of deposition. 



It may be stated generally, with reference to the horizontal 



