DAVIS : THE GKAND CANYON OF THE COLORADO. 163 



manner the Colorado found its way across the Grand canyon district to 

 the broken-down mountains of the Basin range province, and thence 

 southwestward to the sea. The river would thus seem to have been for 

 the most part consequent on the form and slope of the surface at the 

 time of faulting, yet it may have been antecedent to various subordi- 

 nate movements of the crust here and there ; and there is a possibility 

 that it may have been in some greater or less degree developed after the 

 faulting by the retrogressive erosion of west-flowing streams that had 

 been encouraged by a favorable tilting of their courses. McGee has 

 suggested the latter process to explain certain streams in " Papagueria " 

 (Arizona and Sonora), whose growth is thought to have been thus 

 accelerated duriug a time corresponding to what is here called the 

 canyon cycle (c, p. 352), and this process certainly deserves deliberate 

 consideration. 



It should be pointed out that a consequent origin for the Colorado is 

 indicated by the following passage in Powell's report on the Uinta 

 mountains: "At last the movements which began at the commence- 

 ment of Tertiary time succeeded in bringing the whole [Plateau] area 

 not only above the level of the sea, but above the general level of the 

 Basin province itself ; so that while the Basin province was drained into 

 the Plateau province in earlier Tertiary time, in late Tertiary time the 

 drainage was reversed, and the streams of the Plateau province found 

 their way to the sea by passing through the Basin province. ... It is 

 the opinion of Mr. Howell, and I believe also that of Captain Dutton, 

 that this drainage was in some cases reversed along the very channels 

 occupied by the ancient streams which ran from the Basin province into 

 the Plateau lakes" (b, p. 35). There is in this quotation much more 

 suggestion of a consequent origin of the Colorado than is elsewhere to 

 be found in the reports of the observers whose names have been so 

 often cited here : nevertheless, the published reports give no indication 

 of the manner in which the denudation of the upflexed area on the 

 southwest may have prepared the way for the production of a south- 

 westward slope when the district was afterwards broken and heaved by 

 faulting. 



If there be any truth in the suggestion that the Colorado did not 

 cross the Grand canyon district until after the faulting of the plateau 

 blocks, then it was in the cycle of erosion thus initiated (the post-fault- 

 ing cycle) that the great denudation was essentially completed, leaving 

 little more than the stripping of weak strata and the canyon-cutting for 

 the canyon cycle. The retreat of the escarpments on the faulted blocks 



