22 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
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SEDIMENTARY DEPOSITS. 
The hill occupied by the city reservoir on the northern edge of Cripple Creek 
is capped by a coarse conglomerate containing rounded bowlders of granite, gneiss, 
schist, and diabase up to a foot or more in diameter. Pebbles derived from the 
Tertiary eruptive rocks appear to be entirely absent. The bowlders are weathered 
and decomposed, and the deposit at its north end seems to underlie the volcanic 
breccia of Mineral Hill. 
This conglomerate was mapped as breccia by Cross, as the exposures available 
for his examination indicated the presence of some volcanic material. He showed, 
however, ° that the general material of the deposit was entirely different from the 
normal breccia and suggested that it was probably a remnant of a local lake 
deposit. The character of the conglomerate, which since Mr. Cross’s visit has been 
well exposed in a street cuttin'g, indicates that it is a stream deposit. The occur¬ 
rence is of particular interest because it affords definite information concerning the 
original character of the surface of the region before the eruptions of the Cripple 
Creek volcano commenced. 
On Grouse Hill and Straub Mountain occur some rather obscurely bedded 
grits which are remnants of a formerly more extensive deposit. The material con¬ 
sists chiefly of angular and rounded particles of granite, occasionally several cen¬ 
timeters in diameter. Quartz pebbles are abundant and there are scattered pebbles 
of a hard bluish quartzite. The usual color of these grits ranges from dark brown 
to brilliant red or yellow. 
In describing these sediments Cross b referred to their great similarity to the 
upper Carboniferous grits of the Fountain formation. Believing, however, that 
they rested as a younger formation upon the rhyolite of Grouse Hill and Straub 
Mountain, he correlated them with the Miocene High Park lake beds, and deduced 
partly from this correlation the probable late Miocene age of the Cripple Creek 
volcano. 
The relation between the rhyolite and these grits has been studied by Mr. 
Graton, who finds that the rhyolite is intrusive into the grits and the latter are thus 
older than the High Park lake beds. Fragments of rhyolite, it is true, occur in the 
grits near the intrusive contact, but these were probably introduced at the time of 
intrusion of the eruptive rock into the loose, porous deposit. 
The origin and correlation of these grits, including a small mass of similar 
material found on Copper Mountain, in the northern part of the district, is doubtful. 
It is possible that they are remnants of the Fountain formation. But it seems more 
likely that, as suggested by Cross, they are composed of detritus blown from the 
throat of the Cripple Creek volcano at an early stage of eruptive activity, and that 
they have been protected from erosion by the induration consequent on the intrusions 
of rhyolite and phonolite and by cappings of phonolite. 
The deposit seems to have been at least from 200 to 300 feet thick in the vicinity 
of Grouse Hill and Straub Mountain. The accumulation of this thickness of material 
upon what appears to have been in general a plateau surface is difficult to reconcile 
with the supposition that the grits were deposited in a lake. It is more probable 
a Sixteenth Ann. Rept. U. S. Geol. Survey, pt. 2, 1895, p. 101. 
b Op. cit., pp. 53-55, 106-109. 
