188 USIP IEIL (Co IRMOSSIEIOIC 
peaks of the several ranges termed the Park Mountains, was 
once covered by an extension of the formations now found 
beneath the intervening valley, and upturned on the flanks of 
the uplifts. 
As the central cores of granite now rise between seven and 
eight thousand feet above the adjacent border of the Great 
Plains, the refilling of the gorges and valleys excavated in their 
sides, so as to restore what would have been the surface contour 
of the granite had there been no erosion, would produce an 
elongated dome rising at least seven or eight thousand feet above 
the adjacent plain on the east. If we add to the surface of this 
dome the thickness of sedimentary beds now upturned in its 
eroded flanks, its height will be increased by seven or eight thou- 
‘sand feet; this being the approximate average thickness of the 
upturned border of the strata that extend horizontally beneath 
the Great Plains. 
The most severe test of the hypothesis before us, in respect 
to the magnitude of the results reached, is furnished by what is 
known as the Front Range of the Rockies in Colorado and Wyom- 
ing. This range is probably not the result of the wearing away 
of a single dome, but as the central granite core now exposed is 
continuous from Central Wyoming to southeastern Colorado, a 
distance measured along the curve of the outcrop, of about 400 
miles, we will consider it for the present, as an individual uplift. 
The average breadth of the granite is between twenty and twenty- 
five miles. The great elongated dome from which the Front 
Range of the Rockies has been sculptured, would therefore if 
uneroded, have a length in excess of 400 miles, a breadth of 
probably 40 or 50 miles and a height above the level of Denver 
of 15,000 to 16,000 feet. 
This result reached by pressing our hypothesis to its logical 
conclusion—and assuming in order to make the test as severe as 
possible, that the Front Range is a single uplift—is startling, it 
is true, but no more so than the measures of the amount of ero- 
sion that have been obtained in an adjacent region. In the 
Uinta Mountains, Powell finds that the mean thickness of rock 
