396 BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



grains of chalcopyrite. Such copper pitch is also abundant in the 

 Lucky Strike prospect. 



BANNOCK COUNTY 



Dead black lusterless copper pitch inclosing much residual bornite 

 in small grains occurs in ore of the Moonlight mine of the Pocatello 

 Gold and Copper Mining Co., in the Fort Hall district. 



CUSTER COUNTY 



In the Alder Creek district in Custer County copper pitch is abund- 

 ant in the oxidized ores. Its most common occurrence is in irregular 

 cores which on their borders blend into chrysocolla. The substance 

 seems to grade into chrysocolla in many specimens, but its contact 

 with other minerals is sharp. 6 Specimens of typical ore from the 

 Tiger claim contain masses of vitreous black copper pitch inclosing 

 residual bornite and surrounded by chrysocolla. Specimens from 

 the north tunnel of the Empire mine contain pitch like brown mas- 

 sive material inclosing chalcopyrite. Much of the oxidized ore of 

 the Mackay copper deposits is an impure copper silicate which is 

 characteristically brown in color with vitreous to dull luster and con- 

 choidal fracture. It does not look cupriferous. It is in large part 

 amorphous under the microscope and concentric structures commonly 

 show alternating layers of different shades of red and yellow. An 

 analysis of such brown isotropic ferruginous silicate yielded the 

 results given below: 



Analysis oj brown silicate ore, Mackay 



(R. C. Wells, analyst) Per cent 



Silica (Si0 2 ) 29.6 



Copper oxide (CuO) 19. 4 



Water (H 2 0) 17. 9 



Alumina ( AI2O3) etc 5. 6 



Ferric iron (Fe 2 3 ) 24. 



Ferrous iron (FeO) . 2 



Lime (CaO) 1. 9 



Magnesia (MgO) . 5 



Total 99. 1 



The material of the above analysis contained about 4 per cent of 

 the green crystalline chrysocolla, described above, in isolated crys- 

 tals, small aggregates, and narrow layers of closely packed fibers. 7 



FREMONT COUNTY 



Dark brown copper pitch ore varying from dull through waxy to 

 brilliant vitreous luster, inclosing ragged remnants of chalcopyrite 

 occurs in specimens of ore from the Weimer copper mine, Skull 

 Canyon district. 



e J. B. Umpleby. U. S. Geol. Survey, Prof. Paper 97, p. 51, 1917. 

 ? J. B. Umplegy. Idem. 



