DAVIS: THE GRAND CANYON OF THE COLORADO. 119 



stand of the land during all these systematic changes is accepted tem- 

 porarily, but abandoned as soon as need be in order to meet whatever 

 other conditions may be demanded in any particular case. The question 

 here at issue regarding the sculpture of the Grand canyon district is 

 between an essentially single uplift, rapid or slow but continuous, on 

 the one hand, and two uplifts separated by a long period of denudation 

 on the other. 



It is evident that a broad denudation of the upper members of a strati- 

 fied series can be contemporaneous with a narrow trenching of the lower 

 members only if the former are relatively weak and the latter resistant. 

 The plateau series is not so simply arranged, for among the strata that 

 have been broadly denuded are the heavy and resistant Triassic sand- 

 stones which still stand forth in strong and steep cliffs along their line 

 of outcrop north and east of the Grand canyon ; while in the walls of 

 the canyon, especially in the Kaibab section, there are two heavy series 

 of relatively weak strata, the upper Tonto and the lower Aubrey, which 

 have already retreated to partly graded slopes. The upper Aubrey 

 limestone and sandstone and the Red-wall sandstone and limestone, 

 to whose strength the maintenance of the plateau would have to be 

 credited on the hypothesis of a single cycle of erosion, do not appear to 

 possess any extraordinary resistance in excess of that of the heavy strata 

 which make the retreating cliffs north of the canyon ; and the weak 

 members of the higher series, occupying the slopes between the retreating 

 cliffs, do not seem to be notably weaker than the weak strata between 

 the cliff faces in the canyon, — although exception to this statement 

 should be made with respect to the unusually feeble blue clays of the 

 lower Trias. Hence the canyon ought to be much wider than it is, 

 or the northern "terraces" (cliffs) ought not to have retreated as 

 far as they have, if the whole erosion had been accomplished in 

 one cycle. We are thus held to the conclusion that the broad denu- 

 dation of the plateaus must have been far advanced in an early cycle 

 before the incision of the canyon was begun in a later cycle of 

 erosion. 



These considerations lend direct support to Dutton's view that there 

 were two periods of uplift in the Grand canyon district, but it is desir- 

 able that some additional evidence of the verity of this view should be 

 found in the shape of facts that are less immediately related to postulated 

 conditions than are those thus far mentioned. Four groups of facts of 

 this kind may be noted here, and a fifth will be presented in the account 

 of the canyon in a later section. 



