228 CHARLES E. MUNROE 



a cyanide process, gold to the value of $2,000,000 and silver 

 to the value of more than $300,000 were recovered by cyanide 

 processes. Hence by the use of such processes there were 

 produced in the United States, in one year, gold with an esti- 

 mated value of about $8,000,000, and silver with an estimated 

 value of about $600,000. 



Gold, when found in situ, is generally in quartz veins 

 intersecting metamorphic rocks, and to some extent in the 

 wall rock adjoining these veins. It may occur either free or 

 combined apparently with tellurium and perhaps selenium, 

 and it is associated with pyrite (iron disulphide), chalcopyrite 

 (copper iron sulphide), galena (lead sulphide), sphalerite 

 (zinc sulphide), mispickel (iron sulph-arsenide), and many 

 other minerals containing sulphur, antimony, bismuth, co- 

 balt, nickel, the platinum group of metals, etc. On exposure 

 to the atmosphere these minerals may undergo chemical 

 changes, whereby many of their constituents are converted 

 into other compounds, while the metallic gold is set free. 

 Atmospheric action also breaks the veinstone into fragments, 

 which are ground to smaller particles by attrition. When 

 suspended in flowing water the superior density of the gold 

 causes the particles, together with the heavier particles of 

 the vein matter, to settle out, through a natural concentrating 

 action, in an obstructed portion of the stream, forming allu- 

 vial deposits or placers. Through geologic changes such 

 alluvial deposits may again be formed into rock masses, 

 the gold being found in conglomerate rock cemented together 

 by finely divided material, which may be barren or may itself 

 contain gold. When the gold occurs free in quartz or in a 

 weathered vein, it is called a free milling ore; when it is in- 

 timately associated with sulphurets and similar substances, 

 it is called a refractory ore. 



Free gold, as found in nature, is never pure, being alloyed 

 to a greater or less degree with silver, copper, and other metals. 

 It occurs in quartz in the form of strings, scales, or plates, 

 and at times in considerable masses of aggregated crystals; the 

 scales are often so small as to be invisible, to the naked eye, so 

 that quartz showing no color and no evidence of gold on pan- 

 ning may yet yield a considerable percentage on assay. 



