200 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



larger masses of this rock contain numerous rounded fragments of Archean 

 schists, gneiss, and granite. One of the most prominent features is a body 

 of porphyrite, near the summit of the eastern wall of the amphitheater, which 

 can be traced continuously from below as a dark horizontal line. Near the 

 head it sends down a branch across the cliffs to the bottom ; and what is 

 probably a continuation of the upper branch was observed, as already 

 mentioned, in Buckskin amphitheater, on the southeast side of Democrat 

 Mountain. In the bottom of the amphitheater, near its head, is a remarkable 

 dike of porphyrite, from 20 to 50 feet wide, which has a straight northeast 

 and southwest course and cuts through the narrow wall separating this 

 from the amphitheater at the head of English gulch. The main body of 

 the dike is a fine-grained, dark-colored i-ock, more or less impregnated with 

 pyrites. Irregularly contained within its mass is a second body of darker 

 shade, characterized by inclusions of rounded orthoclase pebbles and large 

 crystals or rounded grains of quartz. A specimen of this singular rock is 

 shown in Plate VII (p. 86). It is evidently a later eruption within the mass 

 of the previously existing dike. Both rocks and then' relations are described 

 in Appendix A. 



In the Archean rocks here exposed are found all the types previously 

 described, and from a face of rock at the head of the amphitheater was made 

 the sketch, which is shown in Fig. 1, Plate IV (p. 52), to illustrate the relative 

 ages of the different varieties, normal mica-gneiss being the oldest, which is 

 peneti'ated by the even-grained eruptive granite, and this again in its turn 

 crossed hj veins of white pegmatite. There are evidences of extensive min- 

 eralization, but no ore bodies have been sufficiently opened to afford an op- 

 portunity for systematic study. Parallel with the dike in the bed of the 

 creek, at the head of the amphitheater, was observed a depo.sit of galena, 

 following the wall of one of the larger pegmatite veins. 



In the mass of Mount Arkansas are many eruptive bodies which could 

 not be traced out, although their existence was shown by the numerous 

 fragments in the debris. On a northern spur of the mountain two dikes 

 were seen, one of a quartz-porphyry, allied to the Lincoln type, the other a 

 hori»blende-biotite-porpliyrite. 



