88 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



portions of the rock biotite is quite abundant and some hornblende appears. 

 The microscope shows glass, but no fluid, inclusions in both quartz and 

 feldspar. The groundmass is cryptocrystalline. In general habit it is 

 more like the recent volcanics than the Chalk Mountain rock, and yet, in 

 some parts, it is with difficulty distinguished from a quartz porphyry. 



A third important body of rhyolite is that which forms Black hill, at the 

 southeast extremity of the map. This is a light, often rather pinkish colored 

 rock, of fresh habit and conchoidal fracture. It carries macroscopically 

 two feldspars, smoky quartz, and some biotite. The microscope shows the 

 groundmass to be granular, and that fluid inclusions occur in both quartz 

 and feldspar and glass inclusions in the quartz. From the hand specimen 

 alone the rock would be difficult to distinguish from an earlier quartz por- 

 phyry, but the manner of its occurrence and its relations to the surround- 

 ing rocks leave little doubt that it must be of Tertiary age. 



On the west slope of Empire hill a f-ne-grained, nearly white rock oc- 

 curs below the White Limestone, which is distinctly orthoclastic and con- 

 tains quartz and biotite. The fact that the quartz contains glass and no 

 fluid inclusions points to a Tertiary age, but the occurrence has not been 

 very carefully studied. A similar rock with larger crystals was found in a 

 brecciated material from the Eureka shaft, in Stray-Horse gulch, which it 

 has not yet been possible to account for. 



Trachyte. At the head of Union gulch are small irregular bodies, in 

 granite and White Limestone, of fine-grained, dark-gray rock, full of brown 

 biotite, with small glassy feldspars and some rounded yellowish quartz grains. 

 The microscope shows hornblende and about equal portions of orthoclase 

 and plagioclase. The groundmass is microfelsitic and has a fluidal structure. 

 The quartz grains seem rounded and worn, and are confined to macroscopic 

 individuals, for which reason they are regarded as accidental rather than 

 normal constituents, and as the rock contains only 61.22 per cent, silica it 

 is considered a trachyte rather than a rhyolite. 



ANDESITE. 



The Buffalo Peaks form a double-pointed mountain mass, rising about 

 a thousand feet above the main crest of the Mosquito Range, some ten miles 



