138 REPORT UNITED STATES GEOLOGICAL SURVEY. 



Arfvedsonite. — Occurs in quartz in El Paso County. An analysis 

 luriiishecl Dr. G. A. Koenig the following result :* 



Per cent. 



Si O2 : 49.83 



TiOi 1.43 



Zr Oj 0.75 



Alj O3 trace 



FezOs 15.88 



FeO 17.95 



Mn O 1.75 



Li O^ ^-^^ 



K3O 1.44 



MagO 0.41 



Ignition 0. 21 



97.97 



Argentite. — Colorado Central Mine, Terrible, and other mines near 

 Georgetown j in the No-Name, Caribou, and others at Caribou ; in 

 some of the silver lodes near Nevada ; in the Senator lode of the 

 Hardscrabble district ; in many of the lodes of the San Juan mining 

 region associated with fahlerz and pyrargyrite. At the Silver Star, 

 Moose, and other mines near Fairpla;y. Usually it is found in small, 

 irregular particles or seams, rarely crystallized. Decomposition re- 

 sults in the formation of native silver. 



Arsenopyrite. — Crystallized and massive in the Bobtail and Gunnell 

 mines. Intimately associated with pyrite and chalcopyrite there. 

 Generally auriferous. Together with silver and copper at the Park 

 lode, Bergens ranch. Occurs also in the Priest Mine near Fairplay. 

 With frauklinite on Eio Dolores, Nevada district, Gilpin County. 



Asbestos. — Occurs in small quantities, partly radiated, near Caribou. 



Asphalt. — Found in the White Eiver region. It occurs in veins, is very 

 compact and brittle. Occurs in springs near the summit of the Book 

 Cliffs ; Canyon City. (Loew.) Several of the petroleoid products of 

 Colorado have been termed asphalt. 



ASTROPHYLLITE. — Occurs in quartz on Cheyenne Mountain. Imbedded 

 in quartz in El Paso County. An analysis furnished Dr. G. A. Koe- 

 nig the following result :* 



Per cent. 



Si02 34.68 



TiOa 13.58 



ZrO.2 2.50 



Fp2 O3 6. 56 



AlsOs 0.70 



FeO 26.10 



MnO 3.48 



K2O 5.01 



Na^O 2.51 



MagO 0.30 



CuO 0.42 



TaOs 0.80 



H2O 3.54 



99. 91 



Atacamite. — On Kendall Mountain in some of the argentiferoas lodes 

 near Howardsville. 



AVENTURINE FELDSPAR. — See ORTHOCLASE. 



AvENTURiNE QuARTz. — See Quartz. 



* Proc. Ac. Nat. Sci. Phil., Part i, 1877, p. 9. 



