NATIONAL ACADEMY BIOGRAPHICAL MEMOIRS VOL: VIII 



small relief will in time be produced by the erosion of rain and 

 rivers. Previous to Powell no one had ventured in the theory 

 of land carving by rain and rivers to go beyond what would 

 Uoday be called a late-mature stage in the cycle of erosion 

 namely, the production of valleys between hills or mountains 

 unless one goes back to the brief generalization of the German 

 philosopher, Kant, who a century earlier had recognized that 

 the action of rain and streams must slowly wear down all high- 

 lands and rob the earth's surface of its inequalities ; or to the 

 broad principle of the Scotch geologist, Playfair, who a little 

 later explained that the earth must tend gradually to become a 

 spheroid of rotation by the external action of erosional forces, 

 whatever its original form had been. But Powell is much 

 more thoroughgoing and definite than any of his predecessors 

 He states in his second report, after recognizing the rapid 

 wearing down of highlands: "The degradation of the last few 

 inches of a broad area of land above the level of the sea would 

 require a longer time than all the thousands of feet which 

 might have been above it, so far as this degradation depends 

 on mechanical processes ; . but here the disintegration 



by solution and the transportation of the material by the 

 agency of fluidity come in to assist the slow processes of me- 

 chanical degradation, and finally perform the chief part of the 

 task"_J\Uinta, 196). This passage is of special interest as 

 being the most explicit statement made by Powell regarding 

 the general possibility that normal erosional processes, working 

 on a land-mass long undisturbed, will ultimately reduce the 

 whole surface to a lowland but little above sealevel. His full 

 understanding of the problem is shown when he thus points 

 out the contrast between what would now be called the rapid 

 changes of the youthful stage early in a cycle of erosion and 

 the extreme deliberation of advanced old age at the end of the 

 cycle. 



PLANATION. 



These general results were not left without practical appli- 

 cation. The great plains of erosion revealed by the superb 

 unconformities in the bottom of the Grand Canyon of the 

 Colorado in northern Arizona were evidently regarded as the 



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