12 RESURVEY OF CRIPPLE CREEK DISTRICT. [BULL. 254. 



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 elliptical 

 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 u 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 by Cross to consist mainly of andesitic 

 fragments, although it was recognized that fragments of phonolite are 

 locally abundant. The most characteristic massive rock of 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. 



The general succession of igneous rocks, according to Cross, is as 

 follows: The earliest rocks were andesites containing some orthoclase. 

 Then came a series of allied phonolitic rocks, rich in alkalies and mod- 

 erately rich in silica, together with some andesites. Among them are 

 trachytic phonolite, nepheline-syenite, syenite-porphyry, phonolite, 

 mica-andesite, and pyroxene-andesite. Phonolite was erupted at sev- 

 eral periods. The nepheline-syenite he considered as probably younger 

 than the trachytic phonolite. At the close were intruded a small num- 

 ber of narrow dikes of basic rocks, the so-called basalts, which contrast 

 very markedly with the phonolite. 



MODIFICATIONS OF EARLIER RESULTS. 



In the course of the present investigation the geology of the 

 district has been entirely remapped upon the carefully revised topo- 

 graphic base. The granites, gneisses, and schists have been differen- 

 tiated and outlined in greater detail than was practicable in the earlier 

 investigation. The oldest rocks in the district are muscovite- and 

 fibrolite-schists. These are closely associated with the fine-grained 

 granitic gneisses such as underlie most of the town of Cripple Creek. 

 This gneiss, in the earlier report, was mapped partly as schist and 

 partly as granite. Both gneiss and schist are cut by a reddish granite 

 which occupies a considerable area extending from Anaconda west- 

 ward beyond the limits of the area studied. This granite is well 

 exposed along Cripple Creek in the vicinity of Mound. 



A second type of granite distinguished and mapped is the coarsely 

 porphyritic rock referred to by Cross as the Pikes Peak type of granite. 

 This rock occupies over half of the district and is the prevailing type 



