352 GEOLOGY AND MINING INDUSTKY OF LEADVILLE. 



The groundinass has a mottled appearance in ordinary light through the gathering 

 of exceedingly minute brownish particles about certain centers, but no optical proofs 

 of a radiate structure could be detected. The quartz crystals are surro unded by a 

 zone of similar constitution. Seen in polarized light, the whole groundmas_s seems 

 cryptocrystalline, no isotropic matter being visible. The substances forming it could 

 not be identified, and they seem to be rather needle-shaped or foliate than granular. 

 An alkali determination in this rock gave 3.50 per cent, of potash aud 2.17 per cent, 

 of soda, while the silica was determined at 68.05 per cent., thus confirming the iden- 

 tification as an acid orthoclastic rock. 



OTHER RHYOLITES. 



Rhyolitic tufa in South Park Four miles south of Fairplay and one mile east of 

 the limit of the map is a small outcrop of rhyolitic tufa occurring in the red sand- 

 stones of the Upper Carboniferous [141]. It is of very limited extent and is appar- 

 ently the extremity of an arm reaching out from some of the larger masses of rliy- 

 olite lying to the south or east. It is of a pink color, very light aud porous, aud 

 includes many fragments of sandstone as well as pieces of a still lighter tufa. Glassy 

 feldspar, swarming with delicate glass inclusions, quartz, biotite, and hornblende, can 

 all be recognized. The cementing matter is dull, stained, fibrous, and largely micro- 

 felsite. The tufa contains 70..'i per cent, of silica. 



Dike in the Ten-Mile amphitheater A rock which seems to be related to the Chalk 

 Mountain Nevadite occurs in the amphitheater forming the source of Ten-Mile Creek, 

 just east of Chalk Mountain [139]. It appears as a dike in the Archeau, for the amphi- 

 theater lies immediately east of the great Mosquito fault. On account of decompo- 

 sition of the feldspars, forming a light greenish-yellow mica, the exact parallelism be- 

 tween the two rocks cannot be absolutely established. The macroscopical appearance 

 suggests an intimate relationship. 



Breccia in the Eureka shaft in the Eureka shaft, Stray Horse gulch, near Lead- 

 ville, a brecciated material was found, in which, among other rocks, is a rhyolite con- 

 taining biotite and larger crystals of feldspar than the type from Empire gulch, but 

 with a similar groundmass [204]. The sanidiues abound in glass inclusions, and, 

 besides the quartz, which is not specially abundant, there are aggregates of tridymite. 



QUARTZIFEROUS TRACHYTE. 



At the head of Little Union gulch, south of Leadville, a rock was found travers- 

 ing the Archean and Lower Quartzite in an irregular dike, which must be regarded as 

 a quartz bearing trachyte [142]. Owing to its small ;irea and minor geological signifi- 

 cance, it has not been designated by a distinct color on the map, but has been included 

 under that of rhyolite. 



Its macroscopical appearance is very different from that of any other rock of the 

 region. The color is dark gray, its most prominent constituent being a glistening- 

 brown biotite, with small glassy feldspars aud a number of rounded yellowish quartz 

 grains. Between these is an ill defined, gray groundmass, which is quantitatively 

 much subordinate to the crystalline constituents. None of the crystals exceeds 0.5 cm 

 in diameter. 



