COLLECTIONS IN ECONOMIC GEOLOGY AND METALLURGY. 3 



G () L I). 



Gold Minekals. 



(fOLD. 



Composition: An. Gold, coutaining' silver in varyiuj^: proportions, 

 and sometimes also traces of copper, bismutli, palladium, and rhodium. 



It crystallizes in the isometric system, especially iu octaUedrons and 

 dodecahedrons. Complex and distorted crystals are common, the latter 

 in reticulated and arborescent shapes. Its most common occurrence is 

 in crystals and crystalline aggregates, massive (non-crystalline), in tine 

 and coarse sand, scales or flattened grains, thin plates of considerable 

 size, and in large masses, the largest ever found being the" Welcome" 

 nugget, weighing 2,218 ounces, found at Ballarat, Victoria, Australia. 



It varies in hardness from 2.5 to 3, and when pure its specific gravity 

 is 19.30, but im[)urities may raise it to 1J>.(> or lower it to 15.G. When 

 pure its color is bright golden yellow, but the presence of silver light- 

 ens the color very much. It has a full metallic luster and is very duc- 

 tile and malleable. 



Almost all the gold produced is found in the metallic state, and it has 

 three prominent modes of occurrence : First, placer gold, in water-worn 

 sands and masses from the beds of streams, either ancient or modern, 

 and on the sea shore; second, quartz gold, crystals, and both crystalline 

 arborescent and non-crystalline masses, imbedded in quartz or in cavi- 

 ties in quartz ; third, sulphuret gold, generally invisibly disseminated 

 through various sulphurets, esjiecially pyrite and chalcopyrite, and occa- 

 sionally mispickel. The two latter modes of occurrence are generally 

 associated and graduate into each other, no large masses of quartz be- 

 ing without sulphides and no large masses of sulphides being without 

 quartz. A small amuont of gold is also found imbedded iu other 

 materials, especially slate. 



PLACER GOLD. 



Ten specimens have been selected to illustrate the most common oc- 

 currence of placer gold, beginning with a well-crystallized specimen 

 that has been but little water- worn, and following tlirough various 

 stages, up to a single large nugget, weighing an ounce, which has been 

 much water-worn. l^o. 11 is a sack used to carry gold dust. 



(1) Crystallized, showiug especially the branching fornisof crystallization. .June, 

 1872. Greenwood district, Cedarberg, El Dorado Connty. California. (ofw/O.) 



(2) Containing quartz, from a 2H-onnce mass, showing crystals. September, 1871>. 

 Miller's claim, Deadwood Gulch, Deer Lodge County, Montana. (55472.) 



(8) Dnst, obtained by hand and long-torn washing. October, 1850. Meyer's chiim, 

 Feather Kivcr, California. (.55479.) 

 (4) Dust, from pan-washing. September, 1879. Bingham Canon, Utah. (5.5468.) 



