COLLECTIONS IN ECONOMIC GEOLOGY AND METALLURGY. 23 



(:5) Quartz, staiuod by flecompositioii products, especially copper iniuerals. Bel- 

 mont Miue, Philadelphia district, Nyo County, Nevada. (15428.) 



(4) Galeua and tetrahedrite, showiag decomposition. Highbridgo Miue, Philadel- 

 phia district, Nyo County, Nevada. (1.5439.) 



All the following speciineus are from the Comstock Lode, Storey 

 County, Nevada : 



(5) Free silver, argeutite, stephanite, and other sulphides, witli quartz, both 

 characteristic and stained by decomposition products. Mexican Mine. (154(1:5.) 



(G) Argentite, in quartz, with other sulphides. Rich ore. Hale & Norcoss Mine. 

 (1.5202.) 



(7) Various sulphides, especially argentite and snlph-antimonides of silver, in 

 quartz. Rich ore. California and Consolidated Virginia Mine. (153()4.) 



(8) Various sulphides, especially argeutite, pyrite, and chalcopyrite, in (|uartz. 

 Rich ore. Crown Point Mine. (15223.) 



(9) Quartz, milk white, imperfectly and coarsely crystallized in plates, with 

 argentite deposited between the crystals. Very characteristic. Crown Poiut Mine. 

 (G5319.) 



(10) Decomposed material and quartz, showing small amounts of sulphides. A 

 good representative specimen. Consolidated Virginia Mine. (15191.) 



(11) Quartz with some calcite, carrying sulphides. Average value 170 per ton. 

 A fair representative specimen. Belcher Miue. (15225.) 



(12) Sulphides, argentite, galena, blende, chalcopyrite, with a little ([uartz. Rich 

 ore. OphirMiue. (1.5193.) 



These Comstock ores contain on the average nearly 50 per cent, of 

 their value in gold, but the relative amount of silver by weight exceeds 

 the gold so much that they are classed as silver ores, and are treated 

 by the milling process with the use of chemicals, and well illustrate this 

 character of ores. 



ROASTING MILLING ORES. 



Where silver ores contain so much base metal, as lead or copper, 

 especially in the form of sulphides, as to prevent the silver from amal- 

 gamating, and yet not enough to constitute a smelting ore, or when the 

 silver is in a complex compound, such as the suli^h-autimonides and 

 arsenides, that will not amalgamate, they must be roasted before they 

 will readily yield the metal to mercury. Occasionally a simple roasting 

 is sufflcient, but generally an addition of salt is required in order to 

 convert the silver into chloride. 



These ores generally occur in a quartzose gangue, and while this is 

 occasionally stained by decomposition products it is frequently quite 

 pure and unaltered. Sometimes carbonates, especially the carbonate 

 of manganese, are abundant in the ore. 



To illustrate the occurrence of roasting-milling ores five specimens, 

 together with a complete series from the Ontario Mine, are shown. 



(1) Free silver and argeutite, in quartz, together with an intimate mixture of sul- 

 phides, several of which carry silver; also carbonate of manganese (rhodochrosite). 

 Rich ore. Moulton Mine, Deer Lodge County, Montana. (55.576.) 



(2) Argeutite, in quartz, together with an intimate mixture of sulphides, several 

 of whi'ch carry silver; also cai'bonate of manganese (rhodochrosite). Moultou Miue, 

 Deer Lodge County, Montana. (55561.) 



