38 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
Cow Mountain, and the several projecting shoulders of Pikes Peak. Other emi¬ 
nences like Trachyte, Rhyolite, and Brind mountains may also have had a similar 
form before erosion, attacking them from several sides, had sharpened their 
summits. This truncated topography probably resulted from the long-continued 
action of erosion; the plateau is regarded as a peneplain; its higher summits may 
possibly be remnants of an older and higher plateau. 
EPOCH OF CONSTRUCTION. 
This lofty, rolling granitic plateau became, in middle or late Tertiary time, the 
scene of intense volcanic activity. Violent explosions with almost inconceivable 
force blew a hole through the granite, scattering its debris over the surrounding 
country. A remnant of this detritus is still believed to be preserved at the lowest 
outlet of the prevolcanic drainage on Grouse Hill and Straub Mountain. Repeated 
explosions filled the volcanic neck with breccias, and the result was the upbuilding 
of a volcanic cone on the granitic plateau. Final intrusions of phonolitic magmas 
occurred in the breccia, in the surrounding granite, and even, as on Grouse Hill and 
Straub Mountain, in the granitic debris. The question may now be asked, What 
was the height of this cone? This query has more than academic interest, for 
another highly important question hinges on this point, namely, At what depth 
below the surface were the gold-bearing veins formed ? 
Recognizing the fact that the sections (PI. II, in pocket) of the present topog¬ 
raphy entirely fail to represent the volcanic mountain which must have existed, 
and that near the summits of the Amlcanic hills are intrusive bodies of granular 
rocks, we may conclude that a great part of the accumulated lavas and breccias 
and tuffs have been carried away. The height of the mountain must have been less 
than one-third the diameter of its base. The steepest slopes of large volcanic cones 
are rarely over 30° and toward the base the slopes become very much less steep. 
If the summit attained 15,000 feet—about 5,000 feet above the present highest 
elevation—the base of this cone must have had a radius of at least 5 miles and 
probably very much more. As the prevolcanic surface is reasonably well preserved, 
we should expect to find breccias and tuffs—at least in small patches—scattered 
over the plateau, especially in the slightly eroded basin of West Beaver Creek near 
Gillett. This is not the case. The breccias and phonolite found outside of the 
main area are very clearly due to local eruptions. Hence an original elevation 
of 15,000 feet seems improbable. On the other hand, the phonolite of Grouse Hill 
is evidently an intrusive sheet in the granitic detritus now underlying it, and small 
patches of the same granitic sand are found on the summit of the phonolite. Hence 
we are justified in concluding that a thickness of at least several hundred feet of 
volcanic material once rested on top of this phonolite. From a tentative recon¬ 
struction of the cone as shown in fig. 3, it would seem as if its summit could scarcely 
have exceeded 13,000 feet. On this basis the vicinity of Victor would have been 
covered to a depth of about 2,000 feet and Bull Hill to a depth of about 2,500 feet. 
While these figures are merely tentative, they clearly show that it is very improba¬ 
ble that of 5,000 or 10,000 feet of volcanic material have been eroded—a conclu¬ 
sion that is strongly supported by the occurrence of carbonaceous material in the 
breccia as described on page 31. 
