DAVIS: THE GRAND CANYON OF THE COLORADO. 149 
cline without fracture, and has concluded from this that a considerable 
load of overlying strata must have covered the Kaibab when the flexure 
was produced (d, pp. 60, 64) ; in other words, that the flexure occurred 
sometime before the close of the plateau cycle. 
If this conclusion be correct, the crown of the Kaibab arch must 
have risen a thousand feet or more above a denuded lowland on the 
east and west at the close of the plateau cycle. The strikingly even 
skyline of the Kaibab, as seen from any distant point, is therefore to be 
regarded as indicating a stripped structural (upper Aubrey) plain, and 
not as a part of a once baselevelled surface, afterwards uplifted. The 
exposure of the arch to denudation during the later stages of the 
plateau cycle, as well as through so much of the canyon cycle as has yet 
elapsed, is indicated by the much greater dissection of its surface than 
that of the Kanab, which, as explained above, is best regarded as a 
stripped structural plain of the current cycle. The difference between 
the greater and less dissection of the two plateaus is indicated in 
Dutton’s descriptions, for one is there spoken of as diversified by valleys 
which “cover the entire surface” (c, p. 134), while the other is said to 
be (except for deep canyons) “no more uneven than the rolling prairie 
of Iowa” (c, p. 124) ; and this contrast of form may be easily recognized 
on comparing sheets VIII. and XI. of the Grand Cajion Atlas. Another 
corollary of the early uplift of the Kaibab arch is that an open valley 
must have been eroded across it in the later stages of the plateau cycle. 
If this be true, then it must be further supposed that the rapid widen- 
ing of the new and deep canyon in the current cycle has consumed all 
traces of the earlier valley. 
The Crags of Echo Clifs.—A peculiar feature of the Echo cliffs 
deserves brief mention. In looking along this great escarpment from 
the south, its crest at several points between Tuba and Lee’s Ferry is 
seen to rise in serrated peaks with sharp slopes on the east as well as on 
the west, thus departing from its ordinary tabular form. We at first 
took the sharp serrations to indicate a local increase of dip, and on 
coming abreast of them were much surprised to find the dip practically 
unchanged from its uniformly moderate measure. The front view of 
the escarpment at certain points between the serrated peaks disclosed 
sags in the crest line that could not be explained by any local thinning 
of the cliffmaking sandstones, for they seemed to maintain sub- 
stantially a uniform thickness for tens of miles together. It was there- 
fore suggested that the peaks and sags resulted from the occurrence of 
strike faults with small throw, torn on the slope of the monocline. As 
