138 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
courses came to lie across the lines of displacement because the base- 
levelling process had often reduced hard and soft rocks on the two sides 
of a displacement to much the same level. Subsequent elevation 
permitted the river to entrench itself. Weak rock areas were quickly 
swept away, leaving resistant areas as higher plateaus, the lines of 
displacement usually marking the boundaries between the two. Ac- 
cording to this theory the displacements are older than the river's 
course. From the standpoint of the present cycle, the Colorado is a 
structurally superposed river system, having l>een let down from the 
surface of the former peneplain. 
One object which our party had in view was to test the above 
theories by jjersonal observation in the field. 
Faults of the San Francisco Plateau. 
Under this heading are briefly considered several fault lines which 
we encountered on our way across the San Francisco Plateau, from 
the termination of the plateau in the great escarpment overlooking the 
valley of the Verde River, to the crossing of the Little Colorado River. 
These fault lines we could not trace far, nor study carefully, because of 
limited time. They are of minor importance when compared with the 
great displacements farther north, but indicate a more strongly faulted 
condition of the San Francisco Plateau than is usually ascribed to 
this part of the district. 
Our approach to the San Francisco Plateau from the Basin region 
was by a road which follows up the valley of Oak Creek until near 
the escarpment; here we entered Bear Wallow Gulch (pi. 17, fig. 1), 
a small tributary canon, and made the ascent of the escarpment near 
the head of the gulch. As seen from the Basin region, the scarp is 
wonderfully abrupt, and the angular changes in direction of the cliff 
line suggest the influence of faulting. In the face of the scarp, minor 
faults ai'e occasionally seen. Bear Wallow Gulch, a tributary to Oak 
Creek, appears to be developed along a northeast-southwest fault line, 
and the contact between the Lower Aubrey red beds and the overlying 
crossbedded sandstone is somewhat higher on the northwest side than 
on the southeast. The wagon road makes the steep ascent of the es- 
carpment, or the INIogollon Rim as it is locally called, by a series of 
zigzags, near the head of the gulch. At the first of the westernmost 
