INTERPRETATION OF LAND FORMS 129 



resistless power the sword that cuts so many geological knots 

 volcanic force. The Great Canon of the Colorado would be 

 considered a vast fissure or rent in the earth's crust, and the 

 abrupt termination of the steps of the table lands as marking 

 lines of displacement. This theory though so plausible, and so 

 entirely adequate to explain all the striking phenomena, lacks 

 a single requisite to acceptance, and that is truth." 



With such stupendous examples in mind, the dictum 

 of Hutton seemed reasonable : " there is no spot on which 

 rivers may not formerly have run." 



Denudation by Rivers. 



The general recognition of the competency of streams 

 to form valleys was a necessary prelude to the broader 

 view expressed by Jukes (1862) 22 



"The surfaces of our present lands are as much carved and 

 sculptured surfaces as the medallion carved from the slab, or the 

 statue sculptured from the block. They have been gradually 

 reached by the removal of the rock that once covered them, and 

 are themselves but of transient duration, always slowly wasting 

 from decay." 



Contributions to the Journal between 1850 and 1870 

 reveal a tendency to accept greater degrees of erosion 

 by rivers, but the necessary end-product of subaerial 

 erosion a plain is first clearly defined by Powell in 

 1875. 23 In formulating his ideas Powell introduced the 

 term "base-level," which may be called the germ word 

 out of which has grown the "cycle of erosion," the 

 master key of modern physiographers. The original 

 definition of base-level follows: 



"We may consider the level of the sea to be a grand base- 

 level, below which the dry lands cannot be eroded ; but we may 

 also have, for local and temporary purposes, other base-levels of 

 erosion, which are the levels of the beds of the principal streams 

 which carry away the products of erosion. (I take some liberty 

 in using the term 'level' in this connection, as the action of a 

 running stream in wearing its channel ceases, for all practical 

 purposes, before its bed has quite reached the level of the lower 

 end of the stream. What I have called the base-level would, in 

 fact, be an imaginary surface, inclining slightly in all its parts 

 toward the lower end of the principal stream draining the area 



