MOVEMENTS OF SEA-WATER 



177 



as tin- waves can move are used as weapons of attack, both against 

 the shore and against one another. Masses of rock too large for 

 the waves to move (Fig. 179) are worn by the detritus driven back 

 and forth over them, and in time reduced to movable dimensions. 

 They then become the tools of the waves, and, in use, are reduced 

 still more. Thus bowlders are worn to cobbles, cobbles to pebbles, 

 pebbles to sand, and sand to silt. The silt, held in suspension in 

 agitated water, is carried out beyond the range of breakers, and 



Fitf. 180. Showing blocks similar to those of Fig. 179, but reduced and rounded 

 by wave-action. Shore of Lake Champlain. (Perry.) 



settles in water so deep as not to be agitated to its bottom. Thus 

 one generation of shore bowlders after another is worn out, and the 

 comminuted products come to rest in deeper water. 



The effectiveness of waves depends on their strength and on the 

 concentration of their blows. 1 The average force of waves on the 

 Atlantic coast of Britain has been found to be 611 Ibs. per square 

 foot in summer, and 2,086 Ibs. in winter, but winter breakers which 



1 Willis, Jour, of Geol., Vol. I, p. 481. 



