288 THE BEAUTIES Or NATURE chap. 



raised highest, but those which have suffered 

 least, it follows that if in some cases the 

 course, of the river is due to the direction of 

 the mountain ridges, on the other hand the 

 direction of some of the present ridges is due 

 to that of the rivers. At any rate it is cer- 

 tain that of the original surface not a trace 

 or a fragment remains in situ. Many of our 

 own English mountains were once valleys, 

 and many of our present valleys occupy the 

 sites of former mountain ridges. 



Heim and Riitimfeyer point out that of the 

 two factors which have produced the relief of 

 mountain regions, the one, elevation, is tem- 

 porary and transitory ; the other, denudation, 

 is constant, and gains therefore finally the 

 upper hand. 



We must not, however, expect too great 

 regularity. The degree of hardness, the 

 texture, and the composition of the rocks 

 cause great differences. 



On the other hand, if the alteration of 

 level was too rapid, the result might be 

 greatly to alter the river courses. Mr. 

 Darwin mentions such a case, which, more- 



