154 



TALLETS OF DENUDATION. 



VALLEYS OF UNDTJLATIOK. These are produced by two 

 neighbouring elevations, which, by lifting the strata on each 

 side without occasioning fracture or dislocation, have left 

 a valley between, towards the middle of which their planes 

 are inclined, and of which they form the sides. Their struc- 

 ture will be understood by a reference to the following 

 illustration (fig. 92). 



FIG. 92. 



VALLEYS OF DENUDATION. These and the succeeding 

 class have been formed by the action of water on soft and 

 porous strata, in the same manner as the ravines caused by 

 storms, floods, and torrents. Valleys of this kind are of 

 constant occurrence. It is to such erosions that the changes 



are to be ascribed so constantly taking place in the beds of 

 rivers. The term is usually limited to those valleys where 

 the strata are not far removed from a horizontal position, and 

 where their former continuity cannot be doubted (fig. 93). 



VALLEYS OF ELEYATION. These may be described as 

 owing their origin to the circumstance, that certain beds 

 having been originally raised in the form of a wave, have 

 subsequently been exposed to the denuding action of water, 



