316 GEOLOGY 



are rarely strong enough to hold any but the finest material 

 for any considerable length of time. As transporters of sediment, 

 therefore, ocean-currents are at a great disadvantage as compared 

 with rivers. 



How readily particles of extreme fineness may be kept in sus- 

 pension, and how little agitation is necessary to keep them from 

 sinking, is shown both by experiment and observation. Experi- 

 ment has shown that fine particles of clay require days to settle a 

 foot in still water, and the Challenger found fine sediment derived 

 from the land 400 miles from the coast of Africa. Sediment settles 

 more readily in salt water than in fresh, despite the fact that the 

 former is heavier. This is presumably because the salt diminishes 

 the cohesion of the water. 1 



Deposition by ocean-currents is limited by their transportation. 

 Only where they erode their bottoms do they gather coarse ma- 

 terials, and only in the lee of such places are their deposits coarse. 

 Since the material which they carry is generally fine, it is widely 

 distributed before deposition. 



DEPOSITS ON THE OCEAN-BED 



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

 classes according to the depth of water in which they are made. 2 

 Shallow-water deposits are made in water less than about 100 

 fathoms deep, and deep-sea deposits are 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 

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

 coarser sediments 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 relief 

 of the globe, for it appears to mark, approximately, the junction 



1 The reason usually assigned is the " flocculation" of the fine sediment, 

 but there is reason to doubt this explanation. 



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



