DAVIS : THE GRAND CANYON OF THE COLORADO. 137 



Reasonable as the hypothesis of two cycles of erosion seems to be 

 when all the facts are viewed together, it is difficult to point out rep- 

 resentatives of the " very flat expanse " that was produced when the 

 plateau cycle was interrupted by the uplift that introduced the canyon 

 cycle. Dutton states that the older lavas of the Uinkaret and Shivwits 

 plateaus cover considerable areas of Permian strata, which at the time 

 of the early eruptions must have " constituted the general platform [of 

 the district] in much the same way as the upper Carboniferous now 

 does" (c, p. 107). Details concerning the Permian platforms are at 

 present wanting ; but if future observation shall show that they have 

 no cap of resistant Shinarump sandstone, and that they are as level as 

 is implied in Dutton's descriptions, then it may be fairly inferred that 

 they were lowlands at the time of their burial under the lavas ; for the 

 Permian strata are too weak to permit of the production of a plain of 

 erosion within their mass at a signi6cant height above baselevel, such as 

 might well enough occur on the upper surface of a more resistant formation. 

 Accepting this conclusion provisionally, as the most probable one now 

 obtainable, it is then reasonable to infer that the Permian lowland once 

 extended far and wide, and that it was in fact part of the peneplain to 

 which much of the region had been reduced at the close of the plateau 

 cycle. To-day the peneplain on the Permian is preserved only where it 

 was sheeted with lava ; the Permian floor elsewhere visible north of the 

 broad Carboniferous platform must be referred to the canyon cycle, as in 

 the district southeast of Pipe spring. It will be an interesting matter 

 for some future observer to inquire into the stratigraphic relation of the 

 floors on which the older lavas rest in adjacent plateaus, in order to 

 determine how they bear on the date of the faulting by which the 

 plateau blocks are separated. Much more work on the ground will be 

 needed before this elusive question can be definitely settled. 



Since the uplift by which the canyon cycle was introduced, sufficient 

 time has elapsed for an extensive removal of the weaker Permian strata 

 from the plateau surface, wherever they lay within easy reach of the 

 "wash" that must have been actively revived at the opening of the 

 new cycle : " considerable masses of the Permian were then remaining, 

 which have since been eroded," as Dutton puts it (c, p. 120). Even 

 the resistant upper Aubrey strata, revealed by the stripping of their 

 Permian cover, early in the canyon cycle, have suffered a significant 

 amount of dissection, as seems to be the case over much of the Kanab 

 plateau ; but the dissection here is not so mature as that by which the 

 higher Kaibah is characterized, as will be further considered later. 

 vol. xxxvm. — no. 4 3 



