THE WORK OF THE OCEAN. 



345 



Waves are, therefore, most efficient on bold coasts bordered by broad 

 expanses of deep water. 



The less familiar phases of wave-work are accomplished by hydraulic 

 pressure, compressed air, the use of ice, etc. When the water of a wave 

 is driven into an open joint or a cave, the hydrauHc pressure is great, 

 and if the structure be weak, the rock may be broken. When water 

 is driven with force into a cave, the compression of the air may be great 

 if the wave be high enough to close the entrance. When the water runs 

 out of a cave, the air within may be greatly rarefied, while that above 



Fig. 302. — Showing blocks similar to those of Fig. 306, reduced and rounded by wave- 

 action. Shore of Lake Champlain. The rock is Utica shale. (Perry.) 



exerts its normal pressure. In either case the roof of the cave, if it be 

 weak, may be broken. At certain seasons of the year, especially during 

 the spring, waves make destructive use of the ice which is then breaking 

 up, but it is only in high latitudes that sea-ice is of consequence in this 



