NATIONAL ACADEMY IHOC.RAPTIICAI, MEMOIRS VOL. VIII 



rapid gestures of his empty sleeve: "If the Adirondacks had 

 been uplifted in Cambrian time" as was then generally sup- 

 posed "they would have been worn down over and over 

 AGAIN !" 



The discussion of cliffs of displacement and cliffs of erosion 

 in the Colorado River report is an excellent example of Pow- 

 ell's deductive presentation, evidently based upon observed ex- 

 amples, but systematically extended beyond their reach and 

 admirably illustrated by a series of block diagrams by Holmes. 

 The ideal types thus presented are shown in far greater dis- 

 tinctness than could be reached in any direct view of actual 

 examples. It is well said that "the cliffs of erosion are very 

 irregular in direction, but somewhat constant in vertical out- 

 line; and the cliffs of displacement are somewhat regular in 

 direction, but very inconstant in vertical outline" (Colorado 

 River, 191). This sentence may indeed be taken as one of the 

 best examples of Powell's power in condensed verbal exposi- 

 tion. The migration of divides and the associated beheading 

 of consequent streams during the retreat of cliffs of erosion is 

 recognized (Ibid., 210), but the principle here involved was 

 not developed to its more general application. 



r 



PHYSIOGRAPHIC ESSAYS. 



It is in the pages on the land forms of the Plateau province 

 that one finds some of Powell's best physiographic presenta- 

 tion, much better than in two later essays on "Physiographic 

 Processes" and "Physiographic Features," which he contributed 

 to the series of "National Geographic Monographs" in 1895. 

 These monographs were intended for school teachers of physi- 

 cal geography, who were then, as they are still, in too large a 

 proportion very imperfectly trained in their science, and who 

 therefore needed, as they still need, only elementary essays 

 presenting specific examples in simple language. Whatever 

 Powell's earlier experience in teaching school may have con- 

 tributed to his style of presentation, his later experience as 

 leader of exploring expeditions and as organizer and director 

 of large scientific bureaus did not adapt it to the needs of the 

 readers here appealed to. The simple style in which cliffs and 

 canyons are described in the Colorado River report is re- 



36 



