DAVIS: THE GRAND CANYON OF THE COLORADO. 155 
of adjustment to weak structures than is now seen would have been 
almost inevitable. This opinion is fortified when it is noted that the 
two streams in question are of moderate size, and still more when it is 
seen that they head to the northwest against the retreating Tertiary 
escarpment, thus suggesting that they are now longer and larger than 
they were formerly. Their antecedent origin seems improbable, to say 
the least. 
The Summit Valleys of the Kaibab. — The irregular longitudinal de- 
pressions, including Summit valley and De Motte park, in the highlands 
of the Kaibab plateau, have been regarded as of especially obscure origin. 
Dutton thought them all as the work of a single south-flowing antece- 
dent stream whose waters had been withdrawn in the change from the 
moist Miocene to the dry Pliocene climate, and whose bed had been 
deformed by the local uplift of the Kaibab (¢, pp. 193-195). This view 
seems open to question because of certain improbabilities that it in- 
volves and of certain possibilities that it omits. The association of the 
Summit valley depression with the axis of the Kaibab uplift, a broad 
flat arch, with stronger flexure on the east than on the west, seems too 
close to be the work of chance, — as would necessarily be the case if 
the depression were the work of a stream whose origin antedated the 
uplift. The axial line of the longitudinal depression does not now de- 
scend continuously towards the canyon ; a long narthern stretch (Sum- 
mit valley proper) slopes northward and discharges to the east into 
House-rock valley and thence to Paria river; a more southern por- 
tion (De Motte park) also slopes northward, and, except for shallow 
sinks on its floor, discharges eastward by a deep ravine down the east 
Kaibab monoclines. Two other shorter portions also slope northward. 
Certain intermediate parts slope southward, but they are much shorter 
than the parts just mentioned. Dutton concludes that all these parts 
once had a continuous southward slope, and that the present discon- 
tinuity of slope is due to a reversal of grade by the uplift of the 
Kaibab. 
The continuous southward slope assumed for the depression seems 
open to question, especially when it is remembered that the recession of 
the Triassic cliffs demands a more remote date for the Kaibab arch than 
early Pliocene. The retreat of the heavy Triassic strata around the 
north end of the Kaibab suggests that at least some of the mesozoic 
strata once stretched partly over the uplifted area: they may indeed 
have stretched all over the Kaibab when the uplift was formed, as sug- 
gested by Walcott on account of the absence of fractures in the east 
