84 



ECONOMIC GEOLOGY 



Methods of Placer Mining. In the early history of placer 

 mining, only a few feet of earth next to the bed rock and the up- 

 per surface of the bed rock itself was panned, washed or sluiced, 

 for the richest portion of the entire placer lies near the bed rock. 

 The earliest method of reclaiming the gold was by panning. This 

 was followed by the rocker, the long-torn and sluice-box, the 

 ground sluice, drift mining, the monitor, the hydraulic elevator 

 and the electric dredge. In the hydraulic process, the entire 

 placer is washed by carrying the auriferous gravel into sluices 

 across which riffles are placed for the extraction of the gold. In 



FIG. 63. American Hill Placer mine, Elk City, Idaho. (After W. Lindgren, 

 U. S. Geological, Survey.) 



some cases mercury is placed upon the riffles and the free gold 

 unites with the mercury in the formation of an amalgam. (See 

 Fig. 63.) 



More than one-fourth of the gold mined in California at present 

 is obtained through dredging, mostly from ground previously 

 mined. The electric dredge has solved the problem of mining 

 the gravels below the water level and in rapidly flowing streams 

 (Fig. 64). 



The chief difficulty of placer mining in the Klondike is the 

 permanently frozen ground, which has led to certain peculiarities in 



