316 CONDITION OF THE FLOOR OF THE OCEAN. 



With increasing depth and distance from the continents the depos- 

 its gradually lose their terrigenous character, the particles derived 

 directly from the emerged land decrease in size and in number, the 

 evidences of mechanical action disappear, and the deposits pass slowly 

 into what have been called pelagic deposits at an average distance of 

 about 200 miles from continental coast lines. The materials composin 

 pelagic deposits are not directly derived from the disintegration of the 

 continents and other land surfaces. They are largely made up of the 

 shells and skeletons of marine organisms secreted in the surface water.^ 

 of the ocean, consisting either of carbonate of lime, such as pelagic 

 moUusks, pelagic foraminifera, and pelagic algae, or of silica, such as 

 diatoms and radiolarians. The inorganic constituents of the pelagic 

 deposits are for the most part derived from the attrition of floating 

 pumice, from the disintegration of water-logged pumice, from show- 

 ers of volcanic ashes, and from the debris ejected from submarine vol- 

 canoes, together with the products of their decomposition. Quartz 

 particles, which play so important a role in the terrigenous deposits, 

 are almost wholly absent, except where the surface waters of the ocean 

 are affected by floating ice, or where the prevailing winds have driven 

 the desert sands far into the oceanic areas. Glauconite is likewise 

 absent from these abysmal regions. The various kinds of pelagic 

 deposits are named, according to their characteristic constituents, 

 pteropod oozes, globigerina oozes, diatom oozes, radiolarian oozes, 

 and red clay. 



The distribution of the deep-sea deposits over the floor of the ocean 

 is shown on the map here exhibited, but it must be remembered that 

 there is no sharp line of demarcation between them; the terrigenous 

 pass gradually into the pelagic deposits, and the varieties of each of 

 these great divisions also pass insensibly the one into the other, so 

 that it is often difficult to fix the name of a given sample. 



On another map here exhibited the percentage distribution of car- 

 bonate of lime in the deposits over the floor of the ocean has been 

 represented, the results being founded on an extremely large number 

 of analyses. The results are also shown in the following table: 



The carbonate-of-lime shells derived from the surface play a great 

 and puzzling role in all deep-sea deposits, varying in abundance accord- 

 ing to the depth of the ocean and the temperature of the surface waters. 



