100 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
microcline, and orthoclase are frequent and sometimes very plentiful, and locally 
fragments of schist are seen near the contact, but rarely in the midst of the breccia 
body. Sometimes the rock is evenly granular, but more commonly the larger grains 
are held in a finer-grained groundmass. which is at times so comminuted that the 
highest powers of the microscope detect individual grains only with difficulty. The 
feldspars show some kaolinization, but are otherwise usually little changed. Dark 
silicates of all kinds are almost entirely removed through decomposition, and their 
place is frequently taken by either carbonate or pyrite, which also replace other 
minerals. Practically every specimen holds considerable dolomitic carbonate, and 
it proves to be this mineral which has accomplished most of the cementation and 
induration. A few grains of secondary quartz and chalcedony are sometimes 
observed, and little patches and microbotryoidal concentric coatings partially filling 
cavities are composed of opal. Silicification is, on the whole, however, surprisingly 
slight. The great majority of specimens hold crystalline grains of pyrite formed by 
replacement. Fluorite is found sparingly distributed throughout most of the 
breccia in small grains which have replaced other constituents. In the vicinity of 
mineral veins silica, carbonate, fluorite, and pyrite are apt to be more plentiful. 
RHYOLITE. 
Exposed in small isolated patches on the upper slopes of Grouse Hill is a vol¬ 
canic rock readily determined as rhyolite. To the west of the Cripple Creek district 
the rock occurs in much larger areas which were doubtless originally connected 
and formed an extensive sheet. It is probable that this was largely a surface flow. 
It is considered by Cross to be of Miocene age. Where freshest the rock is cf 
brownish-red color and composed of a compact, earthy groundmass, in which occur 
a few small crystals of sanidine and still less numerous grains of quartz and glisten¬ 
ing flakes of biotite. None of the phenocrysts exceed a size of over 3 mm. In 
most places the rock is bleached to a light yellow and exhibits a noticeable vesicular 
structure which shows the direction of flow. In this weathered condition the 
rhyolite is easily mistaken for the fissile, bleached phonolite which occurs near by. 
Quartz occurs in clear fragmental grains, frequently corroded and often holding 
embayed areas of the groundmass. Many of the phenocrysts of sanidine are well 
formed, and they commonly show Carlsbad twinning. Some of them are likewise 
corroded. A comparatively small amount of microcline is present. Biotite occurs 
in brownish foils, but is frequently decomposed and removed. The predominant 
groundmass is evidently a partially devitrified glass rich in silica. It shows the 
peculiar patterns of the felsophyric texture. In the more decomposed specimens 
the crystallization of the originally amorphous basis has reached such a stage that 
the groundmass may be called microgranular. 
Cross considered that the rhyolite was older than the feldspathic grits of Grouse 
Hill and Straub Mountain, and on this conclusion based his arguments concerning 
the age of the Cripple Creek volcano. Later study has necessitated some modifi¬ 
cations of this conclusion. It now seems probable that the grits have been invaded 
by the later rhyolite and indurated near the contact. The presence of rhyolite in 
them may be explained by brecciation at the time of intrusion. That brecciation 
of the rhyolite has actually occurred can be seen at one place. The contact of 
