300 



GEOLOGY 



success, for the blocks bounded by joints may be loosened and 

 literally quarried out. Waves of clear water, even when their force 

 is many tons to the square foot, have little effect on rock which is 

 thoroughly solid. 



The impact of the waves is generally reinforced by the detritus 

 they carry. The sand, the pebbles, and such stones as the waves 



Fig. 245. Angular blocks of rock which have fallen from the cliff above, 

 as a result of undercutting by the waves; Grand Island, Lake Champlain. 

 The rock is Black River limestone. Although from the shore of M lake 

 instead of the sea, the principles illustrated are the same. (Perry.) 



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. 245) 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 to 

 smaller and smaller size. Thus bowlders aiv reduced 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 



