THE WORK OF THE OCEAN. 367 



ably exist in similar situations, it yet remains true that ocean-currents 

 are on the whole but feeble agents of erosion. 



As agents of transportation, ocean-currents are scarcely more impor- 

 tant than as agents of corrasion, for they transport only what they 

 erode, if the life which inhabits them be left out of consideration. This 

 phase of their work has probably been exaggerated through a confusion 

 of transporting energy and actual transportation. Ocean-currents 

 which do not touch bottom roll no sediment and carry only what may 

 be held in suspension. A river's power of transporting sediment in 

 suspension is due largely to the cross-currents occasioned by the uneven- 

 ness of its resistant bottom (p. 117). If a particle of mud in suspension 

 in a river drops to the bottom, as it frequently does, it may be picked 

 up again and carried forward. If, on the other hand, a particle in sus- 

 pension in an ocean-current once escapes the moving water by setthng 

 through it, the current which does not drag bottom has no chance to 

 pick it up again. Very fine sediment may be carried by an ocean-cur- 

 rent far beyond the point where it was acquired, but currents which do 

 not touch bottom are rarely strong enough to hold any but the finest 

 material for any considerable length of time. As transporters of sedi- 

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

 with rivers. 



How readily particles of extreme fineness may be kept in suspension, 

 and how little agitation is necessary to keep them from sinking, is shown; 

 by the experiments of Sorby, who showed that while a sand grain y|^ ; 

 of an inch in diameter will settle one foot per second in still water, fine 

 particles of clay require days to sink through the same distance. The 

 Challenger found fine sediment derived from the land 400 miles from 

 the coast of Africa, and that not opposite the debouchure of any large 

 river. 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. 



Deposition by ocean-currents is hmited by their transportation. 

 Only where they erode their bottoms do they gather coarse materials, 

 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. 



Ocean-currents have little influence on the configuration of coast- 

 lines. 



