THE WORK OF THE OCEAN. 



343 



of the water is competent also to break up and remove rock which was 

 once resistant, but which has been superficially weakened by changes of 

 temperature. Rock affected by numerous open joints is likewise attacked 

 with success, for by the dash of the waves the blocks between the joints 

 may be loosened and Hterally quarried out. It may, however, be 

 doubted whether the dash of waves of clear water, even when their force 



Fig. 301. — 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 a lake instead of the sea, the principles 

 illustrated are the same. (Perry.) 



is many tons to the square foot, has any appreciable power to wear rock 

 which is thoroughly sohd. 



The impact of the waves is generally reinforced and made effective 

 by the impact of the detritus they carry. The sand, the pebbles, and 

 such stones as the waves can move are used as weapons of attack, 

 being turned against one another and against the shore. Masses of 

 rock too large for the waves to move (Fig. 301) are worn by the detritus 



