t 
ANCIENT CRYSTALLINE ROCKS-GRANITE. 43 
Iii Tertiary time, as a part of the great eruptive activity manifested in this 
general regiori, a volcano broke through these fundamental rocks at about the center 
of the area. The products of this Cripple Creek volcano, together with a minor 
occurrence of a rock of about the same age, but having a different source, constitute 
the second group. 
ANCIENT CRYSTALLINE ROCKS. 
The rocks of the older division embrace granites, gneisses, and schists, and in 
addition a small area of olivine syenite and numerous dikes of diabase which cut 
the first three named above. All are probably of pre-Cambrian age. 
GRANITE. 
The granites of the district may be divided into a number of varieties, each 
distinctly recognizable in the field. They were described by Cross and named, 
respectively, Pikes Peak, Cripple Creek, and Spring Creek types. No attempt to 
separate them on the map was made by him. Dr. E. B. Mathews, who assisted 
Mr. Cross in the mapping of the Pikes Peak quadrangle, has made a somewhat 
detailed study of the granites within that area and has published his resuP: in the 
Journal of Geology, volume 8, 1900, pages 214-240. 
PIKES PEAK GRANITE. 
The Pikes Peak variety, the most extensively developed rock in the district, 
occurs to the north, east, and south of the area occupied by the volcanic rocks and 
for long distances in those directions beyond the limits of the region shown on the 
map. It is a light-pink to red coarse-grained rock, noticeably deficient in dark 
constituents, and much of it rather poor in quartz and hence consisting largely of 
an alkali feldspar which in some cases gives to it a porphyritic appearance through 
the development of more or less idiomorphic columnar individuals up to over an 
inch in length. Many of these large feldspar crystals are so arranged as to give to 
the granite a rather pronounced flow structure, and this drawn-out appearance is 
increased in many places by shearing. At some points, as on Calf Mountain, the 
shearing has been sufficient to transform the granite into a gneiss, with tails or 
schlieren of the quartz, feldspar, and mica fragments, and knots or augen of the 
feldspar phenocrysts. Not uncommonly even these traces of original texture are 
obliterated over small areas, and the equivalence of the resulting even-grained 
much foliated gneiss to the Pikes Peak granite is established only through marginal 
gradations into more massive rock. The extreme degree of foliation is reached 
alone: certain narrow zones where shearing has been so intense that a true schistose 
structure has been developed. Weathering causes a marked disintegration of the 
rock, furnishing a medium-grained angular gravel, composed either of separate 
mineral grains or of small fragments of the rock. This disintegration, with the 
formation of gravel, unquestionably has much to do with the even slopes of many 
of the granite hills. The massive rock is thus covered, and more advanced stages 
of decay are not often observed. 
Under the microscope microcline with its characteristic twinning is seen to be 
generally the most abundant constituent and is usually fairly fresh. Plagioclase 
