DAVIS: THE GRAND CANYON OF THE COLORADO. 13% 
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 considerabie 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 significant 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 
eycle. 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 (e, 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 Kaibab is characterized, as will be further considered later. 
VOL, XXXVIII.— NO. 4 3 
