352 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



The gioimdmass bas a inottled appearance in ordinary light tbrousli 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 surrounded by a 

 zone of similar constitution. Seen in polarized light, the whole groundmass 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 and 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 EHYOLITES. 



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 [Ul]. It is of very limited extent and is appar- 

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

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

 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 tula contains 70.3 per cent, of silica. 



Dike in the Ten-Mile amphitheater A rock which SCCUIS 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 — lu 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 sauidines abound in glass inclusions, and, 

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



QTTARTZIFEROUS TRACHYTE. 



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

 ing theArchean and Lower Qnartzite in an irregular dike, which must be regarded as 

 a quartz-bearing trachyte [1-12]. ■ Owing to its small area and minor geological signifi- 

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

 under that of rhj'olite. 



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 and 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 O.o"'^ 

 in diameter. 



