34 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
of the Vindicator latite-phonolite prevails, but it contains two or three smaller 
masses of syenite which are usually separated from the latite-phonolite by sharp 
contacts, although in some places there is a gradual transition. 
A very persistent dike of latite-phonolite traverses the breccia in the Hull City 
mine, from the massive rock in the Vindicator to some point in the Findley ground. 
The Zenobia and Pharmacist mines show that the large mass of latite-phonolite 
north of Altman is overlain on its western side by breccia. The arm extending from 
the main mass to Bull Cliff is exposed in the Isabella mine where, at a depth of about 
1,000 feet, it contracts rather rapidly to a narrow dike. 
Little is known of the shape of the intrusive masses of Big Bull Mountain, as they 
are unexplored by deep mines. 
Two masses of latite-phonolite are exposed on the south slope of Gold Hill and 
in the workings of the Anaconda mine. The eastern mass was mapped by Cross as 
syenite porphyry, while the western one was represented by two separate dikes of 
andesite. A mass of latite-phonolite of unknown size is cut in a west crosscut from 
the Anaconda tunnel toward the E. Porter Gold King mine, but this body is not known 
to reach the surface. Like the' two occurrences mapped it is apparently intrusive 
into the breccia, although the contact is, as usual, rather indefinite. 
As has been already noted, the syenite usually shows gradations into latite- 
phonolite, indicating that the two rocks are facies of a single intrusion of magma. 
In some places, however, as in the Vindicator mine, the two rocks are in eruptive 
contact, showing that one facies had solidified before the intrusion of the other. It 
is not always clear, in such cases, which is the later rock. 
The peculiar trachydolerite of Bull Cliff is probably an intrusive sheet whose 
upper surface has been uncovered by erosion. It is clearly younger than the latite- 
phonolite and is probably younger than the latest eruption of phonolite; but it 
is older than the basic dikes described in a subsequent section. 
OUTLYING INTRUSIVE MASSES. 
While the intrusive masses (exclusive of small dikes) in the breccia of the vol- 
t 
canic neck are nearly all syenite and latite-phonolite, the outlying intrusions derived 
from the Cripple Creek volcanic center are nearly all phonolite. In most cases, as 
on Pisgah, Rhyolite, Copper, Trachyte, and Straub mountains and on Grouse Hill, 
the phonolite bodies of considerable area seem to have the form of irregular sheets, 
which, however, may be merely the upper expanded parts of plug-like intrusions. 
These sheet-like bodies are sometimes directly connected with dikes. Between 
such sheets and the phonolite dikes later described no sharp distinction can be made. 
The phonolite sheets are rarely associated with important deposits of ore. 
The phonolite mass of Beacon Hill is an eruptive plug filling an elliptical con¬ 
duit in the Pikes Peak granite. As this plug is encircled by productive ore deposits 
it has been exposed at a number of points in the El Paso, Gold Dollar, and smaller 
mines in the vicinity. The contact dips steeply into the hill, the average angle being 
near 75°. If this dip should continue downward at the same angle, the bottom of 
the phonolite would be found at an approximate depth of 2,200 feet. The walls of 
the conduit, however, are likely to become steeper or more irregular at greater depth. 
