GEOLOGICAL STRUCTURE OF THE DISTRICT. 
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general plateau referred to above and the coincidence suggests that this plateau 
is of prevolcanic age. Still better evidence is obtained from 
certain detrital deposits which belong to the volcanic epoch or 
to the one just preceding it. In the city of Cripple Creek, at 
the reservoir, situated on a spur of Mineral Hill, at an eleva¬ 
tion of 9,600 feet, lies a small deposit of nonvolcanic conglom¬ 
erate with large waterworn bowlders of granite, gneiss, and 
diabase (p. 22); it is not more than 50 feet thick and is appar¬ 
ently overlain by the breccia which caps the spur north of the 
reservoir. There is good reason to consider this as pre¬ 
volcanic gravel filling a depression in the general surface of 
the plateau, whose antecedent slopes rise gently on the north, 
east, and west to a height of several hundred feet, while the 
natural outlet of the hollow would have been southward or 
perhaps into the area now occupied by volcanic rocks. 
On Straub Mountain and Grouse Hill rest detrital rocks of 
granitic origin with a maximum thickness of 150 or 200 feet. 
They are poorly sorted, consist mostly of sand and fine gravels, 
and are believed to have been deposited as a result of the 
first explosive action preceding the actual volcanic outflows. 
(See p. 22.) Flat bodies of phonolite are intrusive in these 
sands, and as shown on Grouse Hill were once covered by 
them to an unknown elevation. As shown in fig. 2, which is 
an east-west section through Brind and Straub mountains 
and Grouse Hill, these deposits rest on an even surface gently 
sloping westward and rising more rapidly northward. The 
lowest point of their underlying surface is on the west side of 
Grouse Hill, where its elevation is 8,950 feet. To the south the 
basal granite rises again very gently, as seen from the contour 
lines in the Pikes Peak folio, so that there exists here a dis¬ 
tinct depression, the upper continuation of which most likely 
should be traced through the flat amphitheater of upper Wilson 
Creek and into the volcanic area at Victor between Big Bull 
and Squaw mountains. From here one branch probably con¬ 
nected with the upper amphitheater of Cripple Creek, in the 
center of which lies the gravel deposit referred to above, while 
another one seems to have headed northward, passing by 
Cameron and having its source in the plateau about Gillett. 
To sum up, there is good evidence that the surface of 
this area as it was in the last part of the Tertiary period is 
well represented by the irregular granitic plateau with general 
elevations of 9,500 to 10,000 feet, which, though greatly dis¬ 
sected, now extends to the south and southwest of Cripple 
Creek. The undulating country of Beaver Park plateau about 
Gillett belongs to this plateau. The erosion effected in it by 
the present Beaver Creek probably does not amount to more 
than 100 feet. Above this plateau rose flat-topped hills such as Big Bull Mountain, 
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