126 E. C. ANDREWS. 



The Colorado River at El Tovar, Arizona, flows through 

 a plateau 7,000 feet in height, yet the base of the gorge is 

 only 2,500 feet above the sea. 



Numerous other examples might be cited in illustration. 



In all these examples it may be seen at a glance that 

 the streams are still able to carry their loads as a whole 

 easily over their channel bases when in flood. In each 

 case the channel structures consist of dense rock structures. 

 In other words, provided the rainfall in these regions does 

 not sensibly decrease in amount, the streams under con- 

 sideration will cut their bases much more closely to sea 

 level before they become incompetent to carry their loads 

 as a whole over any given point of the fresh rock structures 

 of their channel bases. 



Transitional stage. — But for all streams a slope of 

 channel base is reached at some time along which the load 

 that is delivered to the main channel cannot be moved as 

 a whole over any specified point of the channel base, even 

 during periods of heaviest stream volume. This stage at 

 which the corrasion of fresh rock structures ceases to be 

 the dominant factor in peneplanation may be called the 

 Transitional Stage. Henceforth weathering and transpor- 

 tation become the dominant factors in land sculpture by 

 erosive processes. 



Width of canons. — Tributary streams cut their way into 

 the plateau at the same time that the main stream does. 

 In the early stages these side streams will be hung up. 1 

 By the repeated branching of such streams, the plateau in 

 the vicinity of the main stream becomes riddled with a 

 network of gorges. In hard rocks, such as granites, the 

 writer has observed that the lips of the canon are about 

 one mile broad when once the canon exceeds a depth of 



1 E. C. Andrews, Proc. Linn. Soc. N.S. Wales, 1906, Vol. xxxi, pp. 419 

 -516. pi. xl, 



