ANCIENT CRYSTALLINE ROCKS-SCHIST. 
51 
SCHIST. 
The foliated rocks of granitic texture have been described in the preceding para¬ 
graphs as gneisses. Besides these, however, there occur in the district areas of 
decidedly foliated and cleavable rocks, the origin of which is not clear, but which can 
be easily mapped and are here described under the name of schist. More specifically, 
they are quartz-muscovite schists. The areas designated as schist on Cross’s map 
contain, besides the above rocks, several masses of what is in this report considered 
as gneiss. 
A microscopic examination would seem to indicate two different tjqres—one 
exceedingly schistose through the development of silvery or purplish muscovite in 
flakes up to a centimeter or more in breadth, the other more dense and massive, gray 
to nearly black, and apparently mica-free. But in reality these two types can not be 
separated; they grade into each other. The denser type is found carrying muscovite 
in increasing amount until it can not be distinguished from the more schistose. That 
this gradation is areal as well as constitutional can be seen in the schist mass north¬ 
east of the town of Cripple Creek, where the northwestern portion is chiefly of the 
denser type and the rock becomes more and more schistose toward the southeast. 
Rocks of this kind form a northwestward-trending belt about half a mile wide 
and 4 miles long, reaching from Cameron to the base of Red Mountain, interrupted, 
however, by areas of later volcanic rock. In a general way this belt separates the 
Pikes Peak granite on the northeast from the gneiss on the southwest. A smaller 
belt of like direction and similarly interrupted reaches from the western slope of Gold 
Hill to the foot of Mount Pisgah, and possibly beyond. This strip roughly marks the 
line between gneiss and Cripple Creek granite. An area of schist surrounded by 
breccia, near Fairview station, on the High Line, may represent a continuation of this 
belt. An uneven narrow band of schist of the same character as the foreg-oino- rocks 
follows the contact bet ween the Cripple Creek and Pikes Peak granites west of Beacon 
Hill. 
The microscope shows that all the rocks have a pronounced schistosity. Their 
most characteristic component is an aggregate mass of sericite, which shows by the 
parallel, almost fluidal arrangement of its minute shreds that the rocks have been 
much sheared. In the denser rocks sillimanite occurs very frequently with the seri¬ 
cite, in a similar way, forming bands, parallel to the schistosity, of small fibers and 
prisms. One or both of these minerals constitute a base or matrix in which are found 
individual grains of other constituents. Both are seen encroaching indiscriminately 
on all the other minerals, regardless of physical character or chemical composition. 
Quartz is often plentiful, occurring as lenses or augen, with numerous smaller 
grains forming tails at each end. In many specimens it is penetrated by needles of 
sillimanite and shreds of sericite, and in some it is in this way almost destroyed. 
Strain shadows and cracks are of common occurrence. A few small grains of 
fresh microcline are noticeable in one or two specimens. Turbid patches, many of 
them nearly destroyed by the advancing sericite, in some cases show albite twinning 
and extinctions corresponding to albite and oligoclase albite, while in others they 
show no twinning and are probably orthoclase. Careful examination by convergent 
polarized light reveals the presence of numerous clear grains of untwinned feldspar, 
