9 
GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
some andesites. Among them are trachytic phonolite, nepheline syenite, syenite 
porphyry, phonolite, mica andesite, and pyroxene andesite. Phonolite was erupted 
at several periods. The nepheline syenite he considered as probably younger than 
the trachytic phonolite. At the close were intruded a small number of narrow 
dikes of basic rocks, the so-called basalts, which contrast very markedly with the 
phonolite. 
Modification of earlier results .—The present investigation indicates some neces¬ 
sary modifications of the earlier report in the way of stronger emphasis on the 
intimate genetic relationship of the rocks. The “phonolite,” “nepheline syenite,” 
“trachytic phonolite,” “syenite porphyry,” and “andesites” of Cross are all very 
closely related and have been found to be in most cases so closely connected by 
intermediate types as to be practically inseparable. None of the massive rocks 
can properly be called andesite, and although it can not be affirmed that andesitic 
fragments are entirely absent from the usually much-altered volcanic breccia, the 
term “andesitic breccia” does not seem applicable to this formation as a whole, 
which is composed chiefly of fragments of phonolitic rocks accompanied locall}' by 
much granite, gneiss, or schist detritus. 
None of the massive rocks erupted from the Cripple Creek volcanic center and 
now preserved 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 the town of Independence to the nearly 
aphanitic phonolite of the smaller dikes and sheets. Most of them come under the 
designations phonolite, latite-phonolite, trachvdolerite, and syenite. 
While the breccia may in a few places rest upon an uneven surface of erosion, 
it occupies in the main a steep-walled chasm extending to unknown depth and 
constitutes a typical volcanic neck. 
The rhyolite of Grouse Hill and Straub Mountain is intrusive into the associated 
grits of this part of the district. The grits can not, therefore, be correlated with the 
Miocene High Park lake beds, and this modifies somewhat the argument from which 
Cross deduced the probable Miocene age of the Cripple Creek volcano. 
Structure of the district .—The pre-Cambrian crystalline complex which forms 
the general plateau of the region was perforated by a volcanic explosion. The deep 
pit thus formed was filled with a breccia composed of fragments of phonolite and 
allied rocks and of granite, gneiss, and schist. The walls of the pit are steep and 
rather irregular. They exhibit local bench-like flattening and in some places over¬ 
hang the breccia. The breccia occasionally shows an indistinct and nonpersistent 
bedding or banding and its constituent particles vary widely in size.' 
The breccia is cut by intrusive bodies of syenite and trachyphonolite, these 
two rocks showing frequent gradations from one to the other. The intruded bodies 
are usually of very irregular shape and have been peripherally shattered to such a 
degree that their contacts with the breccia are very obscure. These rocks generally 
form stock-like masses or thick, irregular sheets. The breccia is also cut by numer¬ 
ous dikes of phonolite, and this rock forms intrusive sheets and masses of consider¬ 
able size in the prevolcanic crystalline rocks. Beacon Hill is a conspicuous example 
of a plug of phonolite in granite. 
