GENERAL GEOLOGY. 
21 
Much difficulty was experienced during the mapping in an attempt to separate 
the rocks described and mapped by Cross as trachytic phonolite, syenite por¬ 
phyry, pyroxene andesite, and augite-mica andesite. The more carefully their 
occurrences were studied, particularly in underground exposures, the more apparent 
it became that these rocks are not distinct, but are slight variants of one magma 
and, in some places, of one eruptive mass. New chemical analyses and the latest 
petrographic work show that this view is correct. The names “ trachytic phonolite,” 
“syenite porphyry,” “pyroxene andesite,” and “augite-mica andesite” have 
accordingly been dropped and the corresponding rocks have been designated “latite- 
phonolite.” Although some of the augite-mica andesite of Cross is sufficiently 
distinct to be called “biotite trachyte,” it has not seemed advisable to map this 
facies as a separate unit. 
On the older map is shown an area of nepheline syenite near the town of Inde¬ 
pendence. Some important changes have been made in the geological boundaries 
in this part of the field, and the rock originally called “nepheline syenite” is in the 
present report described as syenite. A small area of similar rock has also been 
mapped on the north side of Battle Mountain. Nepheline, if present at all in these 
rocks, is a very subordinate constituent. 
The syenite areas are difficult to map, as the rock in many places passes through 
imperceptible gradations into latite-phonolite. 
Cross, in the first Cripple Creek report , a described the rock forming the sum¬ 
mit of Bull Cliff and called attention to its peculiar character. It was mapped by 
him as phonolite. but he stated that it was widely different from the other phono- 
lites and might be a distinct intrusion. Field examination in 1903 showed the rock 
to be a sheet capping the hill and resting upon breccia and latite-phonolite. In the 
Pilgrim tunnel, on the west side of Bull Cliff, the fact that this capping rock is 
younger than a dike of latite-phonolite is clearly shown. 
We classify this rock as a trachydolerite. It apparently constitutes an inter¬ 
esting link, both in time and in chemical composition, between the phonolitic 
rocks and the basic dikes which were the final eruptive products from the Cripple 
Creek volcanic center. 
FORM OF THE VOLCANIC NECK. 
While it may be true that in a few places the breccia rests upon an uneven 
erosion surface of granite, gneiss, and schist, the evidence obtained during the season 
of 1903 shows that the main breccia mass fills a steep-walled chasm of profound 
depth in the fundamental rocks of the region. From the Conundrum mine, on 
the western slope Gold Hill, to Stratton’s Independence mine, on the south slope 
of Battle Mountain, the contact plunges steeply down, with dips ranging in general 
from 70° to vertical. In some places the granite walls of this chasm actually 
overhang the breccia. It is certain that this entire southwest contact represents 
a part of the wall of the great pit formed by the volcanic explosions that produced 
the breccia. In most of the other parts of the contact where evidence could be 
obtained the walls are also steep. The general conclusion reached is that the 
principal breccia mass, with its associated bodies of intrusive rocks, is in the main 
a volcanic neck. 
a Sixteenth Ann. Rept. U. S. Geol. Survey, pt. 2, 1895, p. 37. 
