130 A CENTURY OF SCIENCE 



through which the level is supposed to extend, or having the 

 inclination of its parts varied in direction as determined by 

 tributary streams.) " 



Analysis of PowelPs view has given definiteness to the 

 distinction between "base-level," an imaginary plane, 

 and "a nearly featureless plain," the actual land surface 

 produced in the last stage of subaerial erosion. 



Following their discovery in the Colorado Plateau 

 Province, denudation surfaces were recognized on the 

 Atlantic slope and discussed by McGee (1888), 24 in a paper 

 notable for the demonstration of the use of physiographic 

 methods and criteria in the solution of stratigraphic 

 problems. Davis (1889) 25 described the upland of 

 southern New England developed during Cretaceous 

 time, introducing the term "peneplain," "a nearly fea- 

 tureless plain." The short-lived opposition to the 

 theory of peneplanation indicates that in America at least 

 the idea needed only formulation to insure acceptance. 



It is interesting to note that surfaces now classed as 

 peneplains were fully described by Percival (1842), 26 

 who assigned them to structure, and by Kerr (1880), 27 

 who considered glaciers the agent. In Europe "plains 

 of denudation" have been clearly recognized by Ramsay 

 (1846), Jukes (1862), A. Geikie (1865), Foster and Top- 

 ley (1865), Maw (1866), Wynne (1867), Whitaker (1867), 

 Macintosh (1869), Green (1882), Richthofen (1882), but 

 all of them were looked upon as products of marine work, 

 and writers of more recent date in England seem reluc- 

 tant to give a subordinate place to the erosive power of 

 waves. Americans, on the other hand, have been think- 

 ing in terms of rivers, and the great contribution of the 

 American school is not that peneplains exist, but that 

 they are the result of normal subaerial erosion. More 

 precise field methods during the past decade have 

 revealed the fact that no one agent is responsible for the 

 land forms classed as peneplains; that not only rivers 

 and ocean, but ice, wind, structure, and topographic 

 position must be taken into account. 



The recognition of rivers as valley-makers and of the 

 final result of stream work necessarily preceded an 

 analysis of the process of subaerial erosion. The first 



