292 MINERALOGY 



tions of this simple method, which reduces the cost of handling 

 the enormous quantities of material made necessary. By the 

 hydraulic method in California the cost has been so reduced that a 

 cubic yard of earth can be handled at a cost of two cents. 



The largest nuggets have been taken from the gravels of Australia, 

 one weighing 190 pounds, another 180 pounds. Gold occurs in 

 quartz veins, especially the veins of schists, porphyry, and those 

 rocks high in silica. Less often do the veins of basic rocks contain 

 gold, though it may be found in some limestones and slates. It is 

 probably more soluble in magmas high in silica. 



Gold as a primary mineral has been reported in granite from 

 Mexico ; in pitchstone from Chili ; and small amounts of gold are 

 shown by assay to be contained in granites, syenite, basalt, and 

 diabases of California. 



Gold is soluble in sodium or potassium silicates, ferrous sul- 

 phates and chloride, and the alkali sulphides. The small amounts 

 of gold contained in the country rocks are dissolved by these natural 

 solvents and transported in solution by the percolating waters to 

 veins or cavities where, through a changed physical condition, as a 

 reduction of temperature and pressure, or by contact with a pre- 

 cipitant, they are deposited. As evidence of such concentration, 

 gold is being deposited with the siliceous sinter at the Steamboat 

 Springs of Nevada and at the hot springs of New Zealand, and its 

 presence in sea water has been repeatedly verified. 



Gold in solution is very unstable and is easily precipitated by 

 such agents as organic matter, ferrous salts, metallic sulphides, 

 especially those of iron, copper, zinc, arsenic, and antimony; with 

 the ores of these metals gold is often associated. 



Pyrite is a constant companion of gold in quartz veins, inclosing 

 it within the body of the crystals as inclusions ; the pyrite on oxida- 

 tion is mostly carried away, leaving the gold behind, contained 

 in a porous, rusty quartz, always so pleasing in appearance to the 

 old prospector. 



The largest producing gold mines of the world are on the Rand in 

 South Africa; at Victoria, Australia ; and in the United States. The 

 United States produced, in 1911, $96,233,428 of pure gold, the mint 

 value of which is $20.6718 per ounce troy or $0.6646 per gram. 

 To this production twenty states contributed, of which Colorado, 

 Alaska, California, Nevada, and South Dakota, in order, were the 

 largest producers. 



.Gold is the basis of the world's coinage; that of the United 



