326 GEOLOGY 



they often lose many of their original characteristics, but enough 

 usually remain to tell the story of their origin. 



Deep-sea Deposits 



Contrasted with shallow-water deposits. The deep-sea deposits 

 cover the ocean-bottom below the 100-fathom line. Their area 

 is considerably more than half the earth's surface. The character- 

 istic deposits are muds, organic oozes, and clays, which, in their 

 physical characteristics, are remarkably uniform. Gravels and 

 sands are rarely found. In regions of floating ice, some diversity 

 is introduced from the varied nature of the materials which it trans- 

 ports. Vegetable life is limited to the surface waters, while animal 

 life is present in them and on the bottom, but absent or nearly so 

 in the great middle zone between. The temperature of the bottom 

 is below 40 Fahr. throughout the larger part of the ocean, and 

 subject to little variation. The conditions over the sea-bottom 

 are therefore very uniform. 



In consequence of the slow accumulation of sediment on the 

 deep-sea bottom, the absence of transportation there, and the 

 nature and small size of the particles, many evident chemical re- 

 actions take place, resulting in the formation of many secondary 

 products, such as glauconite, phosphatic and manganic nodules, 

 zeolites, etc. The amount of matter arising from the decomposition 

 and alteration of minerals and rocks increases, relatively, with in- 

 crease of distance from the land. At the same time there is an 

 increase (relative), in all moderate depths, of the remains of pelagic 

 organisms. We thus pass insensibly from deep-sea deposits of a 

 terrestrial origin (terrigenous deposits) near the land, to pelagic 

 deposits, "in which the remains of calcareous and silicious organ- 

 isms, clays, and other substances of secondary origin play the prin- 

 cipal role." l 



The following table 2 shows the relations of the various groups 

 of marine deposits. 



1 Murray, Challenger Kept., Deep Sea Deposits. 



2 Ibid., p. 186. 



