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BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



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corded. They err, as a rule, towards too great 

 regularity of structural features, as in tbe view 

 of Vermilion cliffs of the Paria (Figure 5). As 

 a whole, they are diagrams rather than pictures. 



The Great Denudation. 



Two Cycles of Denudation. — Some time be- 

 fore our excursion I had heard doubts expressed 

 by a competent and critical observer as to the 

 necessity of postulating pauses in the uplift of 

 the region in order to explain the production 

 of the existing topography of the Grand canyon 

 district. These doubts were based on the close 

 association of the general surface of the plateaus 

 bordering the canyon on the north and south, and 

 of the floor of the esplanade, with certain resistant 

 strata ; the first with the upper Aubrey, the second 

 with the Red wall, as indicated in Figure 3. Omit- 

 ting consideration of the esplanade for the present, 

 let us consider the possibility of producing the 

 broadly denuded plateau and the sharply en- 

 ti*enched canyon in one cycle of erosion, introduced 

 by an essentially continuous uplift, without sig- 

 nificant pause during the movement or supplement 

 after its close. 



It is here assumed that the normal progress of 

 erosion in a single cycle demands the relatively 

 rapid deepening of the main valleys by corrasion, 

 and the correspondingly early attainment of a 

 graded slope along the valley bottoms ; a weath- 

 ering of the valley walls, slower at first than the 

 deepening of the valleys but faster afterwards, yet 

 always at a decreasing rate ; an associated but 

 on the whole a slower series of changes along the 

 minor water-courses ; and with the progress of all 

 these processes, a gradual advance of rapidly awak- 

 ening activities to a phase of maximum develop- 

 ment, followed by a much longer phase of relaxation 

 and a very gradual attainment of the remote and 

 ultimate phase of rest. The assumption of a still- 



