THE WORK OF THE OCEAN 315 



: . The Work of Ocean-currents 



As agents of erosion, ocean-currents are not of great importance. 

 Currents which reach the bottom are comparable, in their effects, 

 to rivers of the same velocity and volume; but most ocean -currents 

 do not touch bottom, and therefore do riot 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 current-swept. The abrasive power of the 

 Gulf Stream is known to continue somewhat beyond its narrow 

 channel, for on the Blake Plateau (between the Bahamas and Cape 

 Hatteras). where the water is 600 fathoms and less in depth, "the 

 bottom of the Gulf Stream .... is swept clean of lime and ooze 

 and is nearly barren of animal life." 1 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. 2 In spite of these examples, and of many others 

 which probably exist in similar situations, it yet remains true that 

 ocean-currents are on the whole but feeble agents of erosion. 



Ocean-currents are scarcely more important in transporting 

 than in eroding, for they carry little except that which they erode, 

 if the life which inhabits them is left out of consideration. Currents 

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

 may be held in suspension. A river's power of transporting sedi- 

 ment in suspension is due largely to cross-currents occasioned by 

 the unevenness of its resistant bottom. 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 suspension 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 sedi- 

 ment may be carried by an ocean-current far beyond the point 

 where it was acquired, but currents which do not touch bottom 



1 Agassiz, Three Cruises of the Blake, Vol. I, p. 259. Agassiz would 

 ascribe the Blake Plateau itself to the Gulf Stream, p. 138. See also Am 

 Jour. Sci., Vol. XXXV, 1888, p. 498. 



2 Reade, Phil. Mag., Vol. XXV (1888), p. 342. 



