198 
THE GRAND CANON DISTRICT. 
Kaibab. These magnificent gorges compare well in many respects with 
those which open into the Grrand Canon. They have less depth, but are 
very nearly or even quite as wide. Those which are near the southern end 
of the plateau are in all respects the rivals of those which open into the 
great chasm, being of about the same depth and amplitude. But as we 
proceed northward, and away from the river, their depth suddenly dimin- 
ishes. The reason is obvious: they open upon the Marble Canon platform, 
which is about 3,000 feet above the river, and no mountain goi'ge can be 
excavated lower than, or even as low as, the level upon which it debouches. 
Both series, however, have been excavated nearly to the lowest possible 
depths, and their relative ages can be judged better by the widths than by 
the depths. Allowance must also be made for differences in the conditions 
which favor or retard this kind of erosion. All things considered, a com- 
parison of the two series of amphitheaters leads to the impression that 
the times required to excavate them were not very unequal. Much un- 
certainty of course must attach to such an inference, but it may be said that 
no reason appears at present for assigning a longer period to one series 
than to the other. This would lead us to assign to the monocline the same 
age as that of the Grand Canon— a conclusion identical with that already 
drawn. Although this argument, considered by itself, is so precarious that 
it could not alone justify so important a conclusion, yet when taken in con- 
nection with the whole mass of concordant facts it adds something to the 
volume of cumulative evidence upon which the conclusion is based. 
