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. 
