ANCIENT CRYSTALLINE ROCKS-GNEISS. 
49 
place is approximate only. The rock takes its name from Womack Hill, which 
stands at the eastern edge of the town of Cripple Creek and whose western slope 
is made up of this gneiss. On the old map the rock was not distinguished, but 
was shown partly as granite and partly as schist. 
The origin of the Womack gneiss is not certainly known; it was probably 
derived by pressure from a granitic rock which, to judge from the small amount of 
quartz present in some places, may have been locally of syenitic or monzonitic 
character. There is no evidence that it is related to the Pikes Peak granite, and 
it is certainly older than the Cripple Creek granite, for it is frequently penetrated 
by narrow stringers and dike-like intrusions of the latter, as can be well seen in 
the various railroad cuts near the Short Line station and along the streets in the 
vicinity of the Warren School in Cripple Creek. It seems probable, therefore, 
that it has been derived from a third distinct granite, which, because of the greater 
deformation it has suffered, is thought to be older than either of the others. 
It is a rather dark, brownish-gray rock of medium grain and fairly uniform 
texture, carrying an abundance of red feldspar and deep-brown mica, and appar¬ 
ently not much quartz. It is characterized by a gneissic structure which varies 
from schistosity to a foliation so massive that the hand specimen shows almost no 
banding, and only larger exposures reveal the true foliated nature of the rock. 
Weathering changes the color to a dirty gray-brown, leaves the biotite conspicuous, 
and thereby exaggerates the schistose appearance. 
Under the microscope orthoclase is seen to be the most abundant feldspar. 
Plagioclase is next in importance and is albite or sodic oligoclase. It generally 
shows albite' and in some cases pericline twinning, but is occasionally untwinned. 
Microcline occurs in varying amounts, hut is seldom abundant. Quartz is found 
to be more plentiful than a megascopic examination would indicate. Inclusions 
are almost absent in some specimens, but in others they occur in profusion. As 
in the granites, they are fluid-filled cavities. Undulatory extinction is rather 
common. The feldspars often hold small oriented grains of quartz, while little 
patches of micropegmatite in arborescent patterns are not uncommon. Biotite 
is the prominent ferromagnesian silicate and occurs in irregular large and small 
plates, frequently corroded, and sometimes contorted. Pleochroism from light 
yellow to deep greenish brown is marked, and the absorption parallel to the cleavage 
is so great that many basal sections are practically opaque. The mineral doubtless 
holds a large percentage of iron. Chloritization is common and decomposition 
along the cleavages into epidote and magnetite is not infrequent. Muscovite 
occurs sparingly as an original constituent, and sometimes is clearly derived from 
biotite with the separation of iron oxide. Apatite is always present, and some¬ 
times is comparatively abundant. Titanite varies in amount, but is on the whole 
rather plentiful in very irregular grains. Crystals of zircon are by no means rare, 
and occasionally a minute prism of tourmaline and a small lath of primary epidote 
are seen. The most important accessory is magnetite, which frequently attains 
considerable prominence. In addition to the foregoing constituents, the gneiss 
northeast of Cameron contains abundant sillimanite. 
