46 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
Cripple Creek granite is more readily studied than in the Pikes Peak type. Biot it e 
loses more or less iron, which distributes itself through the partially kaolinized feld¬ 
spar and deepens the red color of the rock. Further change leaves the biotite 
conspicuous though lusterless and gives to the rock a dirty brownish-red color. 
Along phonolite dikes a common decomposition leaves the rock porous through 
removal of much of the quartz and mica. 
A microscopical examination of this rock confirms the conclusion that its 
composition is not very unlike that of the Pikes Peak variety. Microcline is probably 
the most plentiful constituent, and is usually comparatively fresh. In some 
instances, besides the usual crosshatching due to the combination of albite and 
pericline twinning, individuals are twinned according to the Carlsbad law. Occa¬ 
sionally a fine microperthitic intergrowth is present, and the mineral then becomes 
microcline-microperthite, but whether the other component is orthoclase or plagio- 
clase could not be determined. Orthoclase, the next abundant feldspar, is usually 
untwinned and more or less turbid through the development of kaolin and highly 
polarizing aggregates of muscovite flakes. Plagioclase is in general less common 
than in the Pikes Peak type, but is more variable in composition, having been found 
from albite to calcic oligoclase, inclusive. It is twinned polysynthetically and is 
usually turbid like the orthoclase. In a specimen from the Alpha tunnel the 
plagioclase appears to have been the earliest feldspar to crystallize; its grains are 
often corroded, but where inclosed in microcline, as is not uncommon, it has good 
form and the turbid interior is surrounded by an even, narrow border, perfectly 
clear and of slight ly higher extinction angle. A few grains of ordinary microperthite 
are present. Quartz occurs as usual in the granites, in irregular grains of varying 
size and showing inclusions similar to those in the Pikes Peak type. It is sometimes 
present as poikilitic grains in the microcline, and locally shows a tendency toward a 
definite intergrowth. Little patches of typical micropegmatite occur at places. 
The micas complete the list of essential constituents. Biotite, though seldom 
abundant, is the more common, occurring as brown or greenish-brown isolated plates 
of not very high birefringence. A frequent product of alteration is chlorite, often 
associated with epidote and black iron ore. Fully as common is a bleaching of the 
biotite and transformation into muscovite, a process which can be clearlv observed 
in some sections. Muscovite occurs in three ways—secondary from biotite, as just 
noted, secondary from feldspar, and primary. In the second case the aggregate 
sericitic mass resulting from the weathering of much of the feldspar seems to arrange 
itself into shreds and patches having similar orientation, thus producing grains of 
muscovite. In the last case clear muscovite, often corroded, comes up sharply 
against and sometimes penetrates perfectly Iresh biotite of different orientation; it 
also occurs between grains of other components. 
In addition to these principal constituents several minerals are present only in 
grains of microscopic size. Magnetite, apatite, zircon, and titanite occur in rapidly 
decreasing abundance. Small grains of tourmaline are sometimes seen, and show 
pleochroism in strong yellows, greenish brown, and blues. Prismatic crystals of 
brown color and marked pleochroism are more certainly referable to allanite than 
similar grains in the Pikes Peak variety. A few grains of rutile are present. 
