ft 
GENERAL MINERALOGY OF THE DISTRICT. 123 
Quartz. —Quartz is an important constituent of the older rocks, including 
granites, schists, and gneisses, but is generally absent from the younger volcanic 
rocks. It is one of the principal vein minerals occurring in every mine, chiefly 
forming crusts and combs, granular masses, or mammillary forms in the little fissures, 
and, to a minor degree, replacing the constituents of the adjoining rock. It has 
the usual forms of prism and pyramid, but the crystals are commonly small. Many 
of them appear rounded as if corroded. It is frequently intergrown with calaverite. 
In some places a late silicification has taken place, the quartz replacing earlier vein 
minerals, in part celestite. Smoky quartz with distinct dicliroism of carmine 
brown and pure brown occurs on crusts in the Ironclad mine. 
Chalcedony. —This mineral occurs sparingly as a filling between quartz crystals 
and is rather common as an incrustation lining small vugs in the veins. 
Opal .—The hydrated amorphous silica is not uncommon in Cripple Creek, but 
generally represents the last phases of vein-forming action, and forms also during the 
process of oxidation. It is especially abundant in the Zenobia mine as yellow 
masses, often like tangled wires or rods. It is sometimes found in vugs at consid¬ 
erable depth, as on the 1,000-foot level of the Gold Coin vein. Hyalite, a colorless 
opal, occurs in cracks of the Anaconda dike.® In the Victor and Buena Vista veins 
is found a brilliant-red siliceous mineral consisting essentially of 72 per cent silica, 
18 per cent ferric oxide, 1 per cent potash, and 3 per cent water." 
Magnetite (with ilmenite and titanomagnetite).—These are minor constituents 
of schist, diabase, latite-phonolite, basalt, and other rocks. Magnetite also occurs 
in a pegmatite dike in the town of Cripple Creek. 
Specularite. —This has been observed only once as a vein-forming mineral 
inclosed in quartz. It has also been reported to occur in a pegmatite dike on Rhyo¬ 
lite Mountain. In the form of hematite it is probably present in a finely divided 
state in the oxidized rocks. 
Zircon. —Occurs in microscopic form as an accessory constituent in most of 
the rocks. When the roasted ore is concentrated on Wilfley tables in the chlorina¬ 
tion and cyanide mills, a small quantity of white, heavy concentrate is separated, 
which proves to be zircon and doubtless is derived from the rocks in which the ore 
is contained. 
Rutile. —Occurs in microscopic form as a product of alteration of titanite, due 
to vein formation. 
Limonite. —Abundant, in finely divided form, within the oxidized zone; it 
results from the decomposition of both rocks and vein material. 
Psilomelane and wad. —Black manganese minerals are generally distributed in 
the oxidized zone as stains or filling of small cracks and fissures. Occasionally 
they form irregular masses or nodules (Summit and Pharmacist mines, Penrose). 
They are usually soft and sooty, more rarely hard and massive. These minerals 
undoubtedly result from the oxidation of carbonates containing a small percentage 
of manganese, which occur very abundantly in the veins. 
Molyhdite and ilsemannite. —In a prospect on the north side of Battle Mountain, 
near the Comanche Plume tunnel, a bright canary-yellow mineral occurs in the 
a Penrose, Mining geology of the Cripple Creek district, Colorado: Sixteenth Ann. Rept. U. S. Geol. Survey, pt. 2, 1895, 
p. 127. 
