;68 GEOLOGY, 



DEPOSITS ON THE OCEAN-BED. 



Something has aheady been said concerning the sediments which 

 accumulate in the shallow waters along shores; but the area of marine 

 sedimentation is as extensive as the ocean itself, and the deposits must 

 now be reviewed from another point of view. 



Oceanic deposits may be conveniently divided into two chief groups, 

 dependent on the depth of the water in which they are made.^ These 

 groups are (1) shallow-water deposits, made in water less than some 

 such depth as 100 fathoms, and (2) deep-sea deposits, laid down in 

 water of greater depth. The selection of the 100-fathom line as the 

 dividing depth is less arbitrary than it seems, for passing outward from 

 the shore, it is at about this depth that the bottom ceases to be commonly 

 disturbed by the action of currents and waves ; that sunlight and vege- 

 table life cease to be important at the bottom; and that the coarser sedi- 

 ments which predominate along shore give place, as a rule, to muds and 

 oozes. Furthermore, the 100-fathom line (or some line very near it) 

 is an important one in the physical rehef of the globe, for it appears to 

 mark, approximately, the junction of continental plateaus and ocean- 

 basins. Only because the latter are a httle over-full does the water 

 run over their rims, covering about 10,000,000 square miles of the 

 borders of the continents, converting them from land into epicontinental 

 seas. 



Aside from the deposits made by organisms, shallow-water deposits 

 are divisible into two groups — (a) those immediately along the shore, the 

 littoral deposits, and (h) those made between the littoral zone and the 

 100-fathom line. Both are terrigenous. The deep-sea deposits Hke- 

 wise are divisible into two groups, (a) terrigenous deposits formed close 

 to land, and made up chiefly of materials derived immediately from the 

 disintegration of land formations; and (b) the pelagic deposits, made up 

 chiefly of the remains of pelagic organisms and the ultimate products 

 arising from the decomposition of rocks and minerals. The former 

 predominate in the less deep waters relatively near shore ; the latter in 

 the deeper water far from land. The shallow- and deep-water deposits 

 grade into each other in a belt along the 100-fathom line. 



^ Murray. Challenger Report, Deep Sea Deposits, pp. 184, 185. 



