366 GEOLOGY. 



the relative importance of the two processes can only be determined by- 

 detailed study in the field. When it is remembered that the tendency 

 of shore-erosion is to reduce great irregularities of horizontal configura- 

 tion, though not to obliterate small ones if the coast be heterogeneous 

 in composition (p. 353), and that the tendency of shore-deposition is 

 also to regularity, it is clear that the great irregularities of coast-Unes 

 are due neither to shore-erosion nor to shore-deposition, though minor 

 .ones may be due to either. 



THE WORK OF OCEAN-CURRENTS. 



As agents of erosion, ocean-currents are not, in general, 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 not erode it. Where 

 the current agitates the bottom sensibly, as it often does in shallow 

 water, the bottom is abraded, and in the lee of such places it is doubt- 

 less aggraded. Since ocean-currents do not, for the most part, flow in 

 shallow water, their erosive work is, on the whole, relatively shght; 

 but where they are forced through narrow and shallow passageways, 

 their abrasive work may be considerable. Thus the Gulf Stream, 

 where it issues from the Gulf, has a velocity of four or five miles per 

 hour, and its shallow and narrow channel is current-swept. 

 , j A rough test of the abrasive work of an ocean-current is found in the 

 nature of the bottom beneath it. If this be hard, it indicates that the 

 ;;loose sediment on the floor of the ocean has been swept away, while 

 the presence of fine detritus indicates that the current is not wearing. 

 Thus the abrasive power of the Gulf Stream is known to continue some- 

 what beyond its narrow channel, for on the Blake plateau (between 

 the Bahamas a^nd Cape Hatteras), where the water is 600 fathoms and 

 less in depth, 'Hhe bottom of the Gulf Stream ... is swept clean of 

 ■lime and ooze and is nearly barren of animal life." ^ 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 these examples, and of many others which prob- 



^ 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. 



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



