SYNOPSIS OF PART I. 
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4 
CHAPTER III.-DESCRIPTION AND PETROLOGY OF THE METAMORPHIC AND IGNEOUS 
ROCKS. 
The rocks of the district are chiefly of metamorphic and igneous origin. Two 
broad divisions may readily be made: (1) The pre-Cambrian metamorphic and 
igneous basement complex and (2) the products of the Tertiary Cripple Creek 
volcano. 
The metamorphic rocks comprise a much foliated quartz-muscovite-fibrolite 
schist, which was probably derived from sedimentary rocks, and a biotite gneiss 
derived from a granitic rock. Of the ancient igneous rocks two groups may be dis¬ 
tinguished. The first group comprises three varieties of granite, which are closely 
related in composition and have been named, in order of probable age, the Pikes 
Peak type, a coarse-grained quartz-microcline-biotite granite, the most abundant 
rock of the whole region; the Cripple Creek type, a finer-grained rock of similar 
composition, and the Spring Creek type, of medium grain, composed mainly of 
quartz and orthoclase, with a little muscovite. An interesting feature of these 
granites is the presence of fluorite, which is probably original. The second group 
includes the products of differentiation from an olivine-syenite magma. The pre¬ 
dominant rock is composed of microperthite, diallage, and olivine, with accessory 
quartz. The other rocks are an olivine gabbro, with a doleritic contact phase 
which establishes a close relation to the olivine-bearing diabase dikes which cut 
the granite in many parts of the area. 
Tertiary volcanic rocks afford a striking example of the chemically related 
products of a single center of eruption. The most abundant and characteristic of 
these rocks is phonolite. Next in importance is latite-phonolite, which consists 
essentially of soda-orthoclase, sodic plagioclase, sodalite, nosean, analcite, legirine- 
augite, and augite. A syenite rich in alkalies and containing orthoclase, augite, 
and hornblende as essential constituents is the third member. A trachydolerite 
composed of plagioclase, orthoclase, analcite, augite, and biotite is the least abun¬ 
dant of the rocks occurring in irregular intrusive masses. Three varieties of rocks 
occur as dark basic dikes. These are (1) trachydolerite, (2) vogesite, and (3) 
monchiquite. 
These Tertiary rocks are mentioned above in the order of decreasing silica. In 
spite of wide variations in texture, mineralogical character, and chemical compo¬ 
sition an intimate connection or consanguinity exists between all these rocks, of 
which some of the manifestations are the almost invariable appearance of ortho¬ 
clase and analcite as the final products of crystallization and the remarkably 
constant ratio of silica to alumina. All these volcanic rocks have unquestionably 
been derived from the same magma and constitute an excellent example of mag¬ 
matic differentiation within a confined petrographic province. 
Owing to simple structural relations the composition of the average rock of 
the Cripple Creek volcano can be computed and is found to be close to that of 
phonolite. 
