160 W.O.CROSBY — ARCHEAN-CAMBRIAN CONTACT IN COLORADO 



some, we may at least insist that a merely approximate plane is its ut- 

 most limit — a limit toward which it ever tends but which it can never 

 pass ; for the action of surface agencies, and especially of running water, 

 must ever be differential, more concentrated and energetic on some 

 areas than on others, and even the peneplain demands a degree of sta- 

 bility in the earth's crust which many are reluctant to concede. 



On the other hand, marine erosion or the wearing away of the edge of 

 the land by the waves and currents of the sea is, in the long run, diiffer- 

 ential only in regard to the varying character of the rocks — a limitation 



Figure 38. — Diagrammatic Sections (Waleott). 



Illustrating deposition of sediments on a seashore which is being gradually depressed in relation to 



sealevel. 



which, given time enough, ma}^ be completely overcome; and absolute 

 stability of the crust is so far from being essential to the development of 

 a plane surface that we ma}^ say that a slow movement of subsidence 

 presents on the whole the most fiivorable condition. It is not then so 

 much a baseleveling as a planing process. The subsidence constantly 

 minimizes the task and permits the agent (the gnawing edge of the sea) 

 to clear itself of the debris due to its own operation. Waleott 's diagrams, 

 reproduced in figure 38, conve}^ the idea ver}^ clearly, and it is evident 

 that given a rate of subsidence proportional to the hardness of the rocks 

 the result may pass beyond the peneplain, jdelding, theoretically at least, 



