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" (c, 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" (c, 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 ; 



