58 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
of Cripple Creek. The smaller bodies are sometimes also of irregular outline, as on 
the ridge west of Cripple Creek Gulch, but usually appear in definite sheetlike 
forms or dikes. They are abundant in both the volcanic rocks and the outlying 
granite, and exhibit a rude radial arrangement about the main eruptive center. 
Outside the mapped area also the rock occurs at various points within a radius 
of over 7 miles from the summit of Bidl Hill. Besides numerous dikes in the gran¬ 
ite, masses of notable size form the summits of Nipple Mountain, Miter Peak, Little 
Pisgah Peak, the hills south of Straub Mountain, the hill east of High Park, Mount 
Pisgah, and Cow Mountain. Phonolite is also exposed in several places on the 
southwestern slope of Pikes Peak to an altitude of 1 ] ,000 feet. 
Eruptions of phonolite took place at several different times. This rock fur¬ 
nished part of the material of the volcanic breccia and later invaded it as dikes. 
In the granite dikes may be found of at least two ages, and the larger intrusive 
masses may represent still another eruption. 
Appearance .—In a few places outside the district proper these rocks have a 
marked porphyritic texture. In general, however, they are almost aphanitic, with 
only occasional phenocrysts of glassy feldspar up to 1 cm. in size, or of pyroxene 
in small dark needles, or crystals of one of the sodalite group of minerals. When 
fresh they are usually dense and dark gray or greenish, with often a dull greasy 
appearance. Most of them have a platy structure or fissility, which often aids in 
distinguishing them from other varieties, and which in some cases, particularly in 
narrow 7 dikes, becomes pronounced. 
With the exception of the Beacon Ilill mass, which is considerably altered, the 
larger bodies are comparatively fresh. Pew of the dikes, however, are unaffected 
at the surface by the atmospheric agencies. Decomposition effects a bleaching and 
exaggerates the fissility. Incipient weathering causes the rocks to assume a gray¬ 
ish color, and on more complete change they become light yellow" or greenish yellow, 
or at times reddish. Oxide of manganese sometimes imparts to much decomposed 
rocks a brown color distributed either throughout their mass or as a coating along 
cracks and joints. 
Of this prevailing appearance there are one or two noticeable modifications. 
Some of the rocks are mottled by small lighter-colored spots or blotches, usually 
drawn out in the direction of the fissility, and probably caused by a slight decom¬ 
position of the feldspar microlites in those places. In other cases the mottling is 
due to small dendritic patches rich in segirine needles. Textural modifications are 
of two kinds. The most fissile rocks frequently contain small narrow druses, and 
in these cavities sharp trapezohedral crystals of colorless or yellowish analcite may 
often be seen. The second modification, which frequently accompanies the first, 
is caused by the development of megascopic nephelines. This mineral then occurs 
in stout hexagonal columns up to 2 mm. in size, possesses the usual oily luster, and 
is commonly red. Phonolites of this type occur on the w-estern slope of Khyolite 
Mountain, on the hill w^est of the town of Cripple Creek, and on Grouse Hill and 
Straub Mountain. When they weather, nepheline is one of the first constituents to 
be attacked and is readily removed, giving to the resulting mass a characteristic 
pitted surface. 
