02 ON CAVES. 



such a form, I have observed, that in the rock in 

 which it occurs, a rent or cleft extends along the 

 roof of the cave from the mouth inwards into the 

 rock above ; and when the sand and gravel at the 

 bottom of the cave is removed, the same cleft is 

 found to extend from the mouth of the cave inwards 

 into the rock ? If, therefore, this cleft existed in the 

 rock before the cave was formed, the waves, as they 

 dashed at random upon the sea-shore, would carry 

 along with them particles of sand, which, as they 

 penetrated the cleft in the rock, would widen it by 

 their attrition, and would continue to enlarge it, until 

 at length gravel as well as sand would be dashed in 

 by the waves ; and while the particles of sand were 



FIG. 16. 



i' 



penetrating still further into the cleft of the rock, the 



