DAVIS: THE PLATEAU PROVINCE OF UTAH AND ARIZONA. 35 
in the Kaibab have been developed, are now imitated in the red-wall of 
the western part of the canyon in the Kanab plateau; here the progress 
in widening the canyon and in the accompanying sculpture of its walls 
has been relatively retarded by reason of the resistant nature of the 
whole Tonto formation, as has already been pointed out. The rim of the 
esplanade, which overlies and closely follows the pattern of the red-wall 
cliffs, repeatedly advances in great rounded promontories and recedes 
into sharply re-entrant ravines: amphitheaters and cusps of the Kaibab 
type are here unknown at the red-wall horizon. They are, however, 
already fairly well developed in the higher horizon of the resistant 
upper Aubrey beds, whose cliffs enclose the’ esplanade. The Aubrey 
cliffs, sapped by the weak lower Aubrey red beds, have here been worn 
back about as far from the axis of the canyon as the red-wall cliffs have 
been in the Kaibab; and the two are therefore in similar stages of 
development. Asa rule, however, the Aubrey is traversed by too much 
drainage from the Kanab uplands for the development of perfect amphi- 
theaters, although it shows sharp cusps at several points. 
1n the second place, the bench of Tonto sandstone in the Kaibab sec- 
tion exhibits rounded spurs and sharp ravine re-entrants where its dis- 
section has been retarded in virtue of the resistance of the fundamental 
crystalline rocks that underlie the greater part of its extent ; but on 
passing eastward beyond the apex of the ‘* wedge,” to the district where 
the Tonto is underlaid by the relatively weak Unkar series, dissection is 
much further advanced, and here the Tonto exhibits sharp spurs and con- 
cave amphitheaters that closely resemble those of the red-wall in the 
district already described ; while the red-wall itself is reduced to slen- 
der buttes and pinnacles of small area, or altogether consumed. The 
various forms through which the cliff faces run are therefore evidently 
developed in systematic order. 
In the third place, there is no need of a succession of resistant and 
weak strata for the production of the patterns outlined in Figure 11. All 
that is necessary is a mass that shall not possess horizontal diversity of 
structure by which the development of subsequent streams is guided in par- - 
ticulardirections. Given a mass of horizontal or of homogeneous structure 
in which a drainage system with its branchwork of master and minor 
streams is in process of development: its contour lines must assume in 
due order all the patterns given in Figure 11. Not only so; the con- 
tour lines of the sculptured mass at mid-stage in the cycle have a certain 
historic value ; those at lower levels exhibit the pattern from which the 
contours of the higher levels have been evolved ; those at higher levels 
