268— _. THE BEAUTIES OF 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 Riitimeyer 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 nee 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- 
