* 
20 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
now been done. The Pikes Peak granite prevails over a large area in the district 
and is the common rock in the vicinity of Squaw Mountain and Victor. It is 
younger than the gneiss and schist. The Cripple Creek granite also occupies a 
considerable area extending westward from Anaconda beyond the bounds of the 
area studied and is well exposed along Cripple Creek in the vicinity of Mound. It 
cuts the schist and gneiss and is intrusive into the Pikes Peak granite. The Spring 
Creek granitic mass is of comparatively small superficial extent, and its age with 
reference to the other granites is unknown. 
In the northwestern part of the district there has been found and mapped an 
area of a rock which is mainly an olivine syenite, though the mass exhibits remark¬ 
able range and variability in mineralogical composition. The olivine syenite is 
younger than the Pikes Peak granite, but is pre-Tertiary, for numerous diabase dikes 
genetically related to the syenite are covered or intruded by Tertiary eruptive 
rocks. A dike of anorthosite cuts the olivine syenite and is genetically related 
to it in a manner similar to pegmatite dikes in granite. 
It will be seen on looking at the map that the schist, gneiss, Spring Creek 
granite, Cripple Creek granite, and olivine syenite together constitute a wedge- 
shaped area projecting into the Pikes Peak granite from the west. The center 
of volcanic disturbance is near the point of this wedge. 
VOLCANIC ROCKS. 
A number of new chemical analyses of the Tertiary eruptive rocks have been 
made by Dr. W. F. Hillebrand, Mr. George Steiger, and Dr. W. T. Schaller for this 
report, and the petrography of the district has been carefully studied by Mr. L. C. 
Graton with interesting results. That the phonolite, nepheline syenite, trachytic 
phonolite, syenite porphyry, and andesite of Cross are all closely related types 
connected by intermediate varieties appeared highly probable in an early stage of 
the field work. Mr. Graton’s studies, in connection with the chemical analyses, 
confirm this view and show clearly that all the volcanic rocks, including the -basic 
dikes, are merely divergent eruptive facies of one general magma, characterized 
chemically by about 58 per cent silica, a large proportion of alkalies, the soda being 
usually somewhat higher than the potash, a small percentage of lime and magnesia, 
and a certain quantity of combined water. None of the massive rocks would now be 
called andesite. Though it can not be affirmed that andesitic fragments are entirely 
absent from the usually much altered volcanic breccia, none were recognized and 
the term “andesitic breccia” is certainly not applicable to this formation as a 
whole. It would be more accurate to describe it as a phonolitic breccia, although 
in places near the periphery it consists chiefly of particles of the older rocks through 
which the Tertiary eruptives broke. 
None of the massive rocks erupted from the Cripple Creek volcanic center 
and now present in the district show any evidence of having been surface flows. 
They are for the most part intrusive porphyries, ranging in texture, however, from 
the granular so-called nepheline syenite near Independence to the nearly aphanitic 
phonolite of the smaller dikes and sheets. In the breccia of Rhyolite" Mountain, 
however, and in a smaller area of breccia at the south boundary of the district 
mapped, on the east side of the canyon of Cripple Creek, some of the breccia frag¬ 
ments are vesicular. 
