224 
THE GEAND CARON DISTEICT. 
mian strata overspread the region. These are not limited to the few rem- 
nants described on the Uinkaret, but we find the summit of the Permian 
similarly protected by basalt in many widely separated localities. Thus 
the Red Butte south of the Kaibab division of the canon has a basaltic cap 
which seems to be about as ancient as that of Logan. In the San Fran- 
cisco Mountains may be found remnants of the same formation protected 
by lavas, though our knowledge is not yet sufficient to give us any opinion 
as to how great an antiquity should be assigned to those eruptions. No 
doubt they are Tertiary, but whether Miocene or Pliocene is unknown. In 
the valley of the Little Colorado some Permian masses have been similarly 
protected by basalt and still reveal nearly or quite the entire series. The 
Sheavwits Plateau contains these remnants with basaltic coverings more 
abundantly and upon a larger scale than any other plateau. Thus there is 
a general accord of testimony that at the period of the older basaltic erup- 
tions very large bodies of Permian strata lay upon the Carboniferous plat- 
form. In truth, it seems as if the summit of the Permian then constituted 
the surface of the country, just as the summit of the Carboniferous does now. 
The fact that the older basalts wherever found rest upon the same geological 
horizon, viz, the summit of the Permian, suggests to us the further inference 
that the region near the river was then flat and destitute of deep canons and 
valle}^s, such as now exist there, and, therefore, destitute of great hills, buttes, 
or mesas. The meaning of this is a base-level of erosion. The rivers could 
not corrade, because they had reached for the time being their limiting depth 
in the strata. The work of erosion would then be confined to leveling the 
sculptural inequalities without the power to produce new ones or to augment 
the relief of old ones.* This, it is true, looks at first like drawing a very 
broad and rather remote inference from a very slender basis, and would not 
be justified at all if it were not in general harmony with a wide range of 
facts. Many facts take form and coherence around it which would other- 
wise seem mysterious. Let us illustrate. 
The condition of base-level is one in which the rivers of a region can- 
not corrade. As a general rule it arises from the rivers having cut down so 
low that their transporting power is fully occupied, even to repletion. This, in 
*For a brief exposition of the general idea of a base-level see the concluding part of Chapter IV. 
