102 
THE GRAND CASTON DISTRICT. 
the northern side the Permian cliffs, there of unusual magnitude for that 
formation, constitute an unmistakable limit. Upon the eastern side only 
one-half of the boundary line is well marked. The Toroweap Valley serves 
this purpose very satisfactorily so far as it extends, but this valley fades 
out about 22 or 23 miles north of the Grand Canon and thenceforward to 
the northern boundary there is hardly more than a nominal or even im- 
aginary boundary separating the Uinkaret from the Kanah platform. Its 
length from north to south may be roughly stated at 45 to 50 miles, and 
its width at 8 to 12 miles. Of the four plateaus through which the great 
chasm extends it is very much the smallest. But in point of interest it is 
second only to the Kaibab. 
The Uinkaret offers three groups of facts which are useful and import- 
ant for our purposes. They are (1) Basaltic eruptions, (2) the Humcane 
fault, (3) the remnants of Permian strata. They all contribute data which 
may be utilized in the discussion of the physical evolution and history of 
the region. No attempt will be made to draw from them any new light 
upon the general subjects of volcanism, displacement, or stratigraphy, for 
they present very little that is novel, and they could only serve to add a 
few more facts of no uncommon kind to categories which are already replete 
with observations of similar nature. They will be studied here solely with 
an eye to the main questions considered in this monograph— to find out what 
light they shed upon the physical history of the district. Thus considered 
they may prove to be of some value. 
The foundation of the Uinkaret Plateau is the general mass of Car- 
boniferous strata which everywhere constitute the platform of the interior 
spaces of the Grand Canon district. Throughout the greater part of the 
Uinkaret are found many patches of Permian beds overlying the Carbon- 
iferous with apparent conformity. No doubt if the contacts were thoroughly 
exposed we should find many unconformities by erosion without any differ- 
ence of dip. In a very few places contacts of this nature have been detected 
and under the circumstances they cannot well be regarded as exceptional, 
but should rather be looked upon as representing the ordinary method of 
apposition in these two groups of strata. These Permian remnants, with 
few exceptions, belong to the basal members of that series. They are the 
