70 
GILBERT. 
Pikes Peak and Gunnison stations are within the Rocky 
mountains of Colorado, the first being on a high summit, 
the second in a deep valley. The district may be described 
as a plateau 150 miles broad from east to west, consisting of 
several structural ridges or crustal corrugations, each of 
which is divided into minor ridges by deep valleys of ero¬ 
sion. Though many of its peaks stand 8,000 feet above the 
adjacent parts of the interior plain, its average height above 
that datum is only 2,000 to 3,000 feet. Gravity at the two 
stations exceeds the isostatic requirement by 2,300 and 2,200 
rock-feet. The evident suggestion is that the whole Rocky 
Mountain plateau, regarded as a prominence on a broader 
plateau, is sustained by the rigidity of the lithosphere. The 
group of stations in Yellowstone park repeats the suggestion 
for the Rocky mountains of Montana. That upland is about 
80 miles broad, and its average height above Big Horn val¬ 
ley, at the east, and Snake valley, at the west, is between 
2,000 and 3,000 feet. It consists in part of mountain corru¬ 
gations and in part of volcanic accumulations. The three 
stations indicate gravitational excesses of 1,500, 1,800, and 
2,300 rock-feet. In each case the upland may be conceived 
to have originated from the horizontal compression of some 
crustal tract and the consequent upthrust of superficial por¬ 
tions—a process which would result in local excess of matter 
approximately to the full extent of the uplift. Excess would 
continue until the protuberance was removed by erosion. 
The Colorado plateau province has a width, between the 
Rocky mountains of Colorado and the Wasatch plateau, of 
175 miles. Its characteristic is bodily uplift (at various dates 
since Cretaceous time), with subsidiary corrugation. His¬ 
torically and structurally it is intermediate between the 
Rocky mountains and the Great plains, but more nearly re¬ 
lated to the plains. It is drained by the main branches of 
the Colorado river and is undergoing rapid degradation. 
The two gravity stations within it give widely divergent 
indications. At Grand Junction the apparent excess is 
1,400 rock-feet; at Green River there is an apparent defect 
