128 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
Amphibole .-—Hornblende occurs as a constituent of gneiss, latite-phonolite, 
and syenite; also in some basalts. It is usually of dark-brown color; a blue 
amphibole of doubtful relationship has been observed in the phonolite. 
Pyroxene .—Occurs as augite abundantly in diabase, olivine syenite, phonolite, 
latite-phonolite, syenite, and various basic dikes. iEgirine and segirine-pyroxene 
are found in the phonolites. 
Muscovite.— This mineral occurs in the granites, gneisses, and schists of the 
district. It is found as large foils in some pegmatite dikes in Pikes Peak granite, 
here occasionally of possible economic value; also as large foils in some of the schists. 
As a secondary mineral formed by the action of the vein solutions on feldspars, 
nepheline, and many other minerals, it is fairly abundant in the volcanic rocks, 
near the veins. It here takes the form of sericite, but has nowhere been developed 
on the extensive scale seen in many mining districts. 
Biotite. —Occurs as a constituent of the gneisses and granites; it is less abun¬ 
dant in the volcanic rocks. A dark-brown biotite was found as a vein mineral, 
associated with pyrite, in the Dolly Varden vein, Ophelia tunnel. Possibly this 
is the same occurrence mentioned by Penrose from the Ocean Wave claim, which 
we have been unable to locate. A green micaceous mineral allied to biotite is 
found as a product of rock alteration, probably by vein-forming solutions of 
pyroxene in many volcanic rocks. 
Roscoelite. —This light-green vanadium mica, which frequently accompanies 
rich gold ores, occurs in massive form with quartz, fluorite, and calaverite in the 
Mary McKinney mine and probably in small quantities at many other places. It 
is also believed to be present in the Lost Anna vein of the Portland mine, as well as 
in the Ajax and Stratton’s Independence mines. Iq similar association it is known 
from California gold-quartz mines, from eastern Oregon near Sumpter, and from 
the Kalgoorlie mines in Western Australia. 
Chlorite. —Chloritic minerals form rather abundantly during the alteration of 
the volcanic rocks, seemingly both by ordinary decomposition without oxidation 
and by vein-forming waters. 
Serpentine. —This mineral in no place forms large masses, but is noted as a 
product of decomposition of augite and olivine in the volcanic rocks. 
Kaolin. —Regarding this mineral Penrose says:® 
A large quantity of soft argillaceous material is found in the ore bodies in both the granite and the eruptive 
rocks throughout the district, and has evidently been derived largely from the decay of feldspar and other 
aluminum compounds. Sometimes it is stained brown by iron or black by manganese, but very often it is 
pure white. It occurs throughout the mass of many of the rocks, but is most abundant along lines of Assuring, 
sometimes simply filling cracks, at other times in irregular masses, often a foot or several feet in diameter and 
intermixed with the quartz and other vein minerals. Analyses of this material show it to have a very similar 
composition whether derived from the granite or from the eruptive rocks, and in all cases it is essentially a 
kaolin. 
Three analyses of occurrences in various rocks are given to substantiate this 
statement; in one of them, from the Albany tunnel, on the west side of Bull Hill, 
a small percentage of barite is mixed with the kaolin. A specimen from the 
Bobtail vein, Portland mine, analyzed by Hillebrand also gave almost the exact 
a Geology and mining industries of Cripple Creek district: Sixteenth Ann. Rept. U. S. Geol. Survey, pt. 2, 1895, p. 127. 
