ORIGIN OF A GORGE. 



155 



But the most characteristic effect of a cascade is that ceaseless 

 undermining of its base and sides, and consequent rupture of the 

 spout or edge of the fall, which causes the cascade by slow degrees to 

 retire farther and farther up the mountain-side, and produces those 

 deepening portals of impending rocks which so much augment the 

 sublimity of a waterforce. 



This effect may be excellently observed in the Carboniferous lime- 

 stone district of the north of England, where so many streams leap 

 from beds of limestone over perishing shales and sandstones, and 

 rising in foam, sap and undermine the base of a large semicircular 

 cliff, till at length the solid limestone crown gives way, and the insa- 

 tiable river renews its destroying attacks. The same destroying power 

 is seen in many of the Swiss waterfalls, particularly in the numerous 

 falls of the Giessbach. 



Gor 



River 



Fig. 50. Diagram of a Gorge. 



This diagram explains the mode of formation of a gorge (B) by the 

 recession of a waterfall (A). The hard bed (R) dipping in the oppo- 

 site direction to the course of the river, forms a ledge at A, over which 

 the water falls, undermining it by excavating the soft shales (s) ; so 

 that at length part of the ledge falls, and as the waterfall recedes the 



I'ig. 51. Falls of Niagara. 



orge extends. If the hard beds are thin there is rarely a great fall. 

 If the rocks dip in the same direction as the stream, rapids are often 

 produced. 



Lyell ingeniously applied the acknowledged fact of the recession 



