on BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
for this change is the weakening of the upper Tonto strata as they 
extend eastward. In the Uinkaret they are strong enough to act as 
cliff-makers with the overlying red-wall limestones ; in the Kaibab they 
are a heavy series of relatively weak gray and yellowish shales by which 
the red-wall cliffs are undermined. The single bench shown in the cross- 
section of the canyon near Toroweap valley, Figure 10 A, is therefore 
replaced by two benches in the canyon of the Kaibab section, Figure 
10 B, the lower bench being capped by the Tonto sandstone and cut by 
the narrow gorge in the fundamental crystalline rocks ; the upper bench 
being held by the red-wall limestone. The gray slopes of Tonto shales 
between the benches are notable features of the canyon in the Kaibab. 
It is interesting to note that in the eastern part of the canyon in the 
Kaibab, which I had opportunity of seeing in the superb view from 
Cc B A 
Ficure 10. 
Sections of the Colorado canyon at the Toroweap (A), in the middle Kaibab (B), and in the 
eastern Kaibab (C). 
Bissell’s point in 1901, the cross-section is still further changed, as in 
Figure 10C; here the basal crystallines give way to relatively weak 
Algonkian strata, and the bottom of the canyon is a well-opened valley 
with some strips of flood-plain bordering the river here and _ there. 
Further up the river, where the east Kaibab monoclines bend down all 
the basal crystallines and Algonkians beneath the canyon bottom, the 
red-wall limestones close in again to form the Marble canyon. Still 
further up at Lee’s ferry, where the red-wall limestones dip under- 
ground and bring the weak Permians and lower Trias down to the 
river, the canyon is for a short distance replaced by an open valley. 
Then the hard Trias cliffs close in to form Echo canyon. The accord- 
ance of form and structure is thus found to be so close that it seems 
inadmissible to accept the esplanade, a local feature in the western part 
of the canyon, in proof of a pause in the general uplift of the whole 
region. Had such a pause occurred, there should be other evidence of 
it besides the esplanade. 
