THE WORK OF THE OCEAN. 379 



are muds, organic oozes, and clays, which in their physical character- 

 istics are remarkably uniform. In regions of floating ice, greater diver- 

 sity is introduced from the varied nature of the materials which the ice 

 transports, but gravels and sands, comparable to those of shallow water, 

 are rarely found. '^ Tides, currents, and waves produce some mechan- 

 ical effects at the upper hmits of the deep-sea region, but on the whole 

 there is an absence of the phenomena of erosion, and mechanical action 

 would appear to be absent except in the case of submarine eruptions. 

 The depth is too great for sunhght to penetrate, and vegetable life is 

 Hmited to the upper zone. Animal Hfe is present in the same zone and 

 on the bottom, but absent or nearly so in the middle depths. The 

 temperature (at the bottom) is below 40° Fahr. throughout the larger 

 part of the area, and if subject to variation with latitude or change of 

 season, these changes affect only the depths immediately beyond the 

 100-fathom Hne. Throughout the whole region there is a very uniform 

 set of conditions. In the shallow-water and httoral zones, owing to 

 the rapid accumulation and the mechanical effects of transportation 

 and erosion, the effects of chemical modification are not very apparent 

 in the deposits ; but in deep-sea deposits, in consequence of the less rapid 

 rate of accumulation, absence of transport, the nature and small size of 

 the particles, many evident chemical reactions have taken place, result- 

 ing in the formation in situ of glauconite, phosphatic and manganese 

 nodules, zeoKtes, and other secondary products. ^^ ^ With increasing 

 depth and distance from the shore, the character of the deposits under- 

 goes a change. There is less and less material derived directly from 

 the land, and more '^amorphous matter arising from the ultimate decom- 

 position of minerals and rocks, and accompanied, in all moderate depths, 

 by an increase [relative] of the remains of pelagic organisms. We 

 thus pass insensibly from those deep-sea deposits of a terrestrial origin, 

 which we call 'terrigenous,' to those deep-sea deposits denominated 

 'pelagic,' in which the remains of calcareous and siHceous organisms, 

 clays and other substances of secondary origin play the principal role." ^ 

 The following table ^ shows the relations of the various groups of 

 marine deposits. _ 



* Murray, loc. cit. ^ Ibid., p. 186. 



