SHALLOW-WATER DEPOSITS 189 



former is heavier. This is presumably because the salt diminishes 

 the cohesion of water. 



DEPOSITS ON THE OCEAN-BED 



The deposits on the bed of the ocean may be divided into two 

 classes 1 shallow-water deposits, made in water less than about 100 

 fathoms deep, and deep-sea deposits, laid down in water of greater 

 depth. The selection of the loo-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 dis- 

 turbed by the action of currents and waves; that sunlight and 

 vegetable life cease to be important at the bottom; and that the 

 coarser sediments which predominate along shore give place to 

 muds and oozes. Furthermore, the loo-fathom line (or some line 

 near it) is an important one in the physical relief of the globe, for 

 it appears to mark, approximately, the junction -of continental 

 plateaus and ocean-basins. Because the latter are a little overfull, 

 the water runs over their rims, covering about 10,000,000 square 

 miles of the borders of the continental protuberances. 



Aside from the deposits made by organisms, shallow-water 

 deposits are divisible into two groups (i) those immediately 

 along the shore, the littoral deposits, and (2) those between the lit- 

 toral zone and the loo-fathom line. Both are terrigenous chiefly, 

 though chemical and organic deposits occur in both. The deep-sea 

 deposits likewise are divisible into two principal groups, (i) the 

 terrigenous deposits near the land, and (2) the pelagic deposits, made 

 chiefly of the remains of pelagic organisms, and the decomposed 

 products of such other materials as reach the deep sea. 



; 



Shallow-water Deposits 



Littoral deposits. The littoral zone is often defined as the zone 

 between high- and low-water marks, but in common speech, the 

 very shallow water a little farther from the coast-line is generally in- 

 cluded. It is the zone in which sand and coarser materials accu- 

 mulate, though muds are met with occasionally in sheltered estu- 

 aries. Generally speaking, the nature of these deposits is deter- 

 mined by the character of the adjoining lands and the local organ- 

 isms. The heavier materials brought down by rivers or worn from 

 the shore by waves are here spread out by waves and shore-currents. 



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



