CHAP. xii. GEOLOGICAL PRINCIPLES. 119 



rangement of valleys in all high regions, as well as the 

 very common phenomenon of a river crossing the main 

 range of a mountain system by a deep gorge; for this 

 merely shows that what is now the highest part of the 

 range was at first lower than that where the river has its 

 source, but has become higher by the more rapid degra- 

 dation of the lateral ranges, owing to their being formed 

 of rock which is more easily disintegrated. The various 

 peculiarities of open valley and narrow gorge, of sloping 

 mountain side or lofty precipice, of rivers cutting across 

 hills, as in the South Downs and at Clifton, when open 

 plains by which they might apparently have reached the 

 sea are near at hand, may be all explained as the results 

 of those simple causes which are everywhere in action 

 around us. It was Sir Charles Lyell who first convinced 

 the whole scientific world of the efficacy of these familiar 

 agents; and the secure establishment of this doctrine 

 constitutes one of the great philosophical landmarks of 

 the nineteenth century. 



The Glacial Epoch. 



The proof of the recent occurrence in the north 

 temperate zone of a glacial epoch, during which large 

 portions of Europe and North America were buried in 

 ice, may, from one point of view, be thought to prove 

 that other agents than those now in operation have acted 

 in past ages, and thus to disprove the main assumption 

 of the uniformitarians. But, on the other hand, its 

 existence has been demonstrated by those very methods 

 which Sir Charles Lyell advocated the accurate obser- 

 vation of what nature is doing now; while an ice age 



