Davis — Notes on the Colorado Canyon District. 257 



lower blocks " (1. c, 191). Secondly, the Vermilion cliffs of 

 the Trias make a great detour around the northern end of the 

 Kaibab arch, such as could not be accounted for if the uplift of 

 the Kaibab were as recent as the erosiou of the canyon. Thirdly, 

 the Trias margin along the Echo cliffs is in close accordance 

 with the monocline of the same name ; this proves that time 

 enough must have elapsed since the bending of the monocline 

 for the Triassic sandstones to retreat from whatever irregular 

 front they may have had while they still lay horizontal, to 

 their present well-defined structural alignment. The time 

 that sufficed for the erosion of the canyon by the active Colo- 

 rado does not seem long enough for the recession of the Tri- 

 assic cliffs from an irregular to a regular front under the attack 

 of dry-climate weather alone. Fourthly, the relatively small 

 retreat of the Aubrey cliffs from the fault lines, as along the 

 Hurricane and the Toroweap faults, may be explained by the 

 relatively recent uncovering of these structures, long ago 

 faulted. 



The origin of the drainage system. — Both Powell and 

 Dutton attribute an antecedent origin to the larger members of 

 the drainage system- in the plateau region. It is possible to 

 conceive of the conditions necessary for the maintenance of 

 such a drainage system through all the movements of upheaval 

 and displacement that the plateau region has suffered ; but it 

 seems to me doubtful if these necessary conditions have been 

 actually provided ; and it is very difficult to find decisive 

 proofs of an antecedent origin. During the great denudation by 

 which thousands of feet of Mesozoic strata were stripped from 

 the plateaus, there must have been opportunity for many spon- 

 taneous rearrangements of river courses. This opportunity 

 would have been increased if there had been several succes- 

 sive cycles or partial cycles of denudation, separated by move- 

 ments of elevation, as is entirely possible: the opportunity 

 would have been still further increased if, as is eminently 

 probable, displacements with slight tilting occurred during the 

 denudation, for, as Hayes has shown, the greater rearrange- 

 ments of drainage are dependent rather on gentle tilting move- 

 ments of elevation than on inequality of rock resistance. 

 The denudation of the region is usually dated as beginning 

 in early Eocene time, because it is postulated that late Eocene 

 strata once stretched all over the canyon district ; but if a true- 

 scale section be drawn, it does not appear improbable that the 

 canon district may have been slightly uplifted so as to undergo 

 some denudation while the Eocene deposits were accumu- 

 lating in basins on the north and east. The whole stretch of 

 Tertiary time, with repeated uplifts and displacements, would 



