1 88 WORK OF THE OCEAN 



and Gulf coasts of the United States (Figs. 174, 187, and PI. XV, 

 Fig. i). It is to b*e noted, however, that in the simplification of 

 the shore-line through deposition, the initial stages may result 

 in great irregularity (Figs. 187 and 191). 



Ocean-currents 



Ocean-currents are due primarily to winds. As agents of ero- 

 / sion, they are not of great importance. Currents which reach the 

 bottom are comparable, in their effects, to rivers of the same ve- 

 locity and volume; but most ocean-currents do not touch bottom, and 

 therefore do not erode it. Only where they flow through narrow 

 and shallow passageways is their abrasive work considerable. Thus 

 the Gulf Stream has a velocity of four or five miles per hour where 

 it issues from the Gulf, and its shallow and narrow channel is cur- 

 rent-swept. Other illustrations of the erosive power of currents 

 have been noted near Gibraltar in water 500 fathoms deep, and 

 between the Canary Islands at depths of 1000 fathoms. In spite 

 of such examples, it yet remains true that ocean-currents are on 

 the whole but feeble agents of erosion. They are scarcely more 

 important in transporting, for they carry little except that which 

 they erode, if the life which lives in them is disregarded. Currents 

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

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

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

 evenness of its resistant bottom. If a particle of mud suspended 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 

 suspended in an ocean-current once escapes the moving water by 

 settling 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-current far from the point where it was acquired, but cur- 

 rents which do not touch bottom are rarely strong enough to carry 

 any but the finest material. 



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 



