GENERAL GEOLOGY. 19 
upon the uneven surface of the plateau. The eruptive rocks of the Cripple Creek 
district are the products of one of the smaller isolated volcanic centers of this 
period, a center characterized by the eruption of phonolite, which does not occur 
elsewhere in this general region. The most voluminous products of the Cripple 
Creek volcano now preserved are tuffs and breccias. They occupy a rudely ellip¬ 
tical area in the center of the district about 5 miles long in a northwest-southeast 
direction and about 3 miles wide. According to Cross, these breccias and tuffs 
rest in part upon an earlier flow of andesite," but mainly upon an unevenly eroded 
surface of the granites and schists, although along the southwest edge of the area 
the contact was found to be so steep as "to support the idea that the central vent 
or vents of the volcano were adjacent to this line.” The breccia is much indurated 
and altered, but was thought to consist mainly of andesitic fragments, although 
it was recognized that fragments of phonolite are locally abundant. The most 
characteristic massive rock emanating from the Cripple Creek volcano is phonolite, 
which was erupted at several periods and more abundantly than any other type. 
It occurs as dikes and masses, not only in the breccia, but in the surrounding granitic 
rocks. Among the massive rocks distinctly younger than most of the breccia 
Cross distinguished trachytic phonolite, nepheline syenite, syenite porphyry, mica 
andesite, pyroxene andesite, and basaltic rocks of various types. The nepheline 
syenite he considered as probably younger than the trachytic phonolite, while the 
basaltic dikes represent the last eruptions in the district. 
MODIFICATION OF EARLIER 1IFST FTS. 
In the course of the present investigation the geology of the district has been 
entirely remapped upon the carefully revised topographic base. A comparison 
of the new map with the old will reveal a number of changes in the boundaries of 
igneous masses, the division of the granitic terrane into several distinct formations, 
the recognition of a number of eruptive masses not shown on the older map, and 
considerable change in the nomenclature of the igneous rocks. There is also 
recorded an important revision of conclusions as to the age and the relation to the 
rhyolite of the sedimentary rocks of Grouse Hill and Straub Mountain. 
BASEMENT ROCKS. 
The most ancient rocks in the district are fibrolitic muscovite schists and fine¬ 
grained granitic gneisses. The gneisses are typically exposed in the streets of 
Cripple Creek and at the terminal station of the Colorado Springs and Cripple 
Creek District Railway. The schists may be well seen in Poverty Gulch between 
the Abe Lincoln mine and the railway trestle bridge and near the station of the 
Florence and Cripple Creek Railroad in Cripple Creek. This gneiss v^as not separa tely 
shown on the older map, being included partly with the granites, which were mapped 
as a unit, and partly with the schist. 
Cross and Mathews recognized three types of granite in the Cripple Creek 
region, which they designated the Pikes Peak, Cripple Creek, and Spring Creek 
types. They did not, however, distinguish these types upon the map. This has 
a At the time Cross wrote, the term “andesite” embraced rocks, such as latite, which have since been given separate 
names. 
