PHYSICAL CAUSES AND EFFECTS. 35 



when gentle and silent as the classic Liris, wore 

 away their banks and triturated into sand and 

 pebbles the solid beds over which they rolled. 

 The lakes, too, dashed their waves against their 

 banks, forming beaches more or less extended, 

 while they were themselves either gradually 

 filled up by the substances carried into them by 

 the rivers, or drained by the gradual depression 

 of their outlets. The surges of the ocean also, im- 

 pelled by the winds, broke against opposing rocks, 

 wearing them down, and then suffering the disin- 

 tegrated materials to subside to the bottom and 

 form those strata subsequently laid bare ; the 

 lightning struck against the cliffs, and hurled them 

 downwards ; the rain percolated through their 

 cavities, and exposed them to the powerful action 

 of the oxygen in the air ; and the volcano and the 

 earthquake raised new islands from the abysses of 

 the ocean, and caused tracts of land to subside 

 beneath its waters. 



The brief span over which human life extends, 

 and the consequent limited range of individual 

 observation, are apt to lead us to suppose that 

 the present state of things is less liable to those 

 modifications which have taken place in former 

 eras of the world's history. But that such a sup- 

 position is erroneous, there is abundant evidence 

 to prove. Even during a single lifetime many 

 changes are perceptible. Those places in the 

 smaller lakes at which the rivers which supply 

 them empty themselves, are frequently perceived 

 in the course of a few years to grow shallow and 



D 2 



