138 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
The graded Permian surface in the northern part of the Kanab plateau 
to-day may, therefore, be better explained as having been in large part 
newly developed with respect to existing local baselevels (such as sills 
of upper Aubrey at the heads of branch canyons) instead of as having 
been preserved with little modification since the close of the previous 
cycle. I am consequently inclined to dissent from Dutton’s opinion 
that “ the broad and slightly varied expanse ” of the surface of denuda- 
tion which “ cuts the strata in such a way that the [low] hills usually 
consist of lower Permian strata lying horizontally, while the shallow 
valleys expose the Carboniferous ” (ce, p. 118) is a close representative 
of the broad lowland of denudation produced at the close of the plateau 
cycle. Truly, “the mean position of the surface of denudation is very 
nearly coincident with the dividing horizon between those two forma- 
tions”: and if “we successively visit districts considerably apart, we 
shall find in one of them [the southern] that the mean position of the 
surface of denudation is below that horizon so far that no Permian 
rocks appear in the hills; in the other [the northern] it is so far above 
it that no Carboniferous rocks appear in the valleys” (ec, p. 118) ; truly 
also the gentle bevelling of slightly inclined strata over large areas is 
often accepted as evidence of peneplanation, as by Campbell and Men- 
denhall for the plateau of West Virginia (p. 483), and by Philippson for 
the plains of Russia (p. 40); but in the plateaus of the Grand canyon 
district another interpretation seems permissible. It should be noted 
that only two formations are referred to in the above-quoted extracts 
from Dutton’s report; the lower one (Carboniferous) resistant; the 
upper one (Permian) relatively weak. These formations rise gently 
southward. The great Carboniferous platform (upper Aubrey) on either 
side of the canyon in the Kanab district, here chiefly under considera- 
tion, is free from Permian remnants over all those large areas whose 
drainage is actively tributary to the main river; while the continuous 
Permian cover, apart from certain large patches that are protected by 
lava sheets, is found only some distance north of the canyon, where the 
dip of the strata brings the Permian down to a level that is safe from 
erosion for the present, and that can be attacked only as the wet- 
weather streams cut deeper into the Aubrey sills on their way down 
branch canyons to the Colorado. Now, if the Permian were the resist- 
ant and the Carboniferous were the weak formation, or if the next over- 
lying and resistant formation further north (Triassic) were also bevelled 
down to the Permian level, the bevelled surface would necessarily be 
regarded as a peneplain, at present uplifted and in process of dissection ; 
wr 
