52 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
probably for the most part orthoclase, with cleavage sometimes distinct . Muscovite 
is generally rather abundant, especially in the more foliated types, as irregular, ragged 
flakes which often appear to have resulted from recrystallization of the sericite 
base, while in several instances they are seen to have been derived from biotite. Bio- 
tite itself occurs in considerable amount in a few specimens and sparingly in some 
others, and at times gives evidence of its former abundance through the presence of 
pseudomorphie areas of sericite and strongly pleochroic chlorite, with the excess of 
iron separated out along the original cleavages as magnetite. 
In some of the darkest, least schistose rock from the area south of Red Mountain 
there occur, along with abundant sillimanite, numerous grains of a mineral which has 
been identified as corundum. It forms hexagonal prisms up to 1 mm. long and half 
as broad, terminated sometimes by the unit pyramid. Sometimes it forms hexago¬ 
nal plates 0.8 mm. across, and frequently it occurs in irregular grains. An imperfect 
rliombohedral cleavage is visible in some individuals. At times a patchy but dis¬ 
tinct pleochroism in blue is seen in parts of a crystal. The refractive index is notice¬ 
ably high. Isotropic sections give a uniaxial figure of negative character. The 
double refraction, as compared with that of other minerals in the section, appears 
somewhat higher than the recorded value for corundum, but owing to its excessive 
hardness the grains of this mineral are found to be considerably thicker than the rest of 
the slide, and as a result its interference colors are proportionally increased. Prisms 
and irregular grains, as well as ill-defined, radiating patches, of epidote are often 
observed. Apatite is common in some specimens, and not infrequently small parti¬ 
cles of zircon are seen. Both these minerals are present in broken or contorted 
grains. Magnetite is usually very plentiful in large and small grains, which at times 
take the form of octahedrons. 
Where feldspar and biotite are found sillimanite appears to be in process of 
formation, occurring in aggregates of very minute fibers radiating in all directions, 
with larger prisms in the interior of the mass. It appears to be an intermediate 
product in the process which finally results in the formation of the extremely 
schistose muscovite rocks. In such specimens, too, there are bands of quartz 
and feldspar grains which exhibit sometimes true granitic texture and at others 
typical cataclastic structure. 
Cross regarded the schists that he found in the course of his study of the Pikes 
Peak quadrangle as, for the most part, inclusions of exceedingly metamorphosed 
ancient sediments, but bethought that some, if not all, of these particular foliated 
rocks of the Cripple Creek district have been produced from Archean gneisses. A 
study of the occurrence of these rocks and of their character as revealed by the 
microscope would point to the same view, but chemical evidence obtained later 
makes it appear probable that the schists were not derived from granitic gneisses. 
The following partial analysis by W. T. Schaller of a representative muscovite schist 
suggests, rather, by the low silica and high alumina, potash, and water, a relation 
to the phyllites or clay slates. The material was collected near the mouth of the 
Cripple Creek and Gold Hill tunnel. 
