THE MENDIPS : A GEOLOGICAL KEVERIE. 



249 



were not then so hard as we now tiud them to be and had 

 not been consolidated into sandstones, grits, and crystalline 

 limestones. And as they rose stop by stej) they were sub- 

 jected to the ceaseless action of the forces of denudation. 

 The waves of the sea cannot have played much part in this 

 denudation ; it must have been in the main sub-aerial. 



Two processes of sub-aerial denudation are now going on 

 over the surface of the British Isles, a superficial scouring 

 of the wliolo area by the action of rain and frost and 

 atmospheric disintegration, and a linear trenching of the 

 land along the special lines of i-iver drainage. The surface 

 so acted on is composed of rocks of differing hardness and 

 powers of resistance to denuding processes ; and according 

 to their method of formation and method of upheaval these 

 rocks are arranged with a greater or less amount of sym- 

 metry. In some places the harder and softer bands alternate 

 with some regularity, and the symmetry is of a compara- 

 tively simple type. This, for example, is the case in the 

 neighbourhood of Bristol and to a large extent in the 

 Mendips. But in others, the Highlands of Scotland for 

 instance, the symmetry is of a far more complex type. In 

 any case the essential fact to grasp is this : that whereas 

 the linear trench formed by a river is down-cut to the same 

 depth in hard and soft rocks, the superficial scouring is far 

 more effective on the less resistant than on the more resistant 

 strata. While the softer rocks are being uniformly lowered 

 by general denudation, the hai-der strata will become fretted 

 with deep ravines and gorges through the linear denudation 

 of the streams and rivers. 



Applying these general principles to our sj)ecial case, we 

 see in the Mendip Hills the nuclear heart laid bare to our 

 view by the scalpel of denudation by which the overlying 

 bods have been removed. The range that we see is only the 



