54 BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



rushes to new fields and the only mining for long intervals was that 

 carried on by the patient Chinese. Many of the camps have since 

 become the sites of extensive modern dredging enterprises. Some 

 relatively rich deposits have remained unworked for a variety of 

 reasons — scarcity of water, depth of bedrock, lack of drainage, large 

 size of boulders, shortness of season, or inaccessibility. The available 

 placers of sufficiently high grade to be worked by ordinary methods 

 are, however, pretty well exhausted. 



The sands of Snake River contain very finely divided gold in wide 

 distribution. This gold has been mined here and there from the 

 Wyoming line to Oregon. While practically all of the gravels along 

 the river are auriferous, most of the deposits contain so little gold 

 that they can hardly be called placers. Some small deposits yield 

 sufficient to pay for extraction if worked on a large scale and a very 

 few may be worked by hand methods. The problem of saving this 

 gold has been much studied without a practical method being found. 

 The gold is exceedingly finely divided, it requiring fully 1,500 grains 

 or " colors" to weigh 1 cent in value, or approximately 3,000,000 of 

 the grains to weigh one ounce, yet, under the microscope, each grain 

 may be seen to be an individual nugget showing rounding and abrasion 

 marks. The particles are often coated or spotted with a crystalline 

 film of silica, making it necessary to grind the gold in a pan before it 

 will amalgamate freely. In size the particles range within relatively 

 narrow limits and there is no gradual shading from the finer particles 

 into impalpable dust, 10 These Snake River bars have been mined 

 in Ada, Blaine, Bingham, Cassia, and Owyhee Counties. 



In composition the gold of both the veins and placers is variable, 

 the variation being in the amount of silver with the gold, there being 

 a transition into electrum, an alloy containing equal molecular parts 

 of gold and silver as discussed below. The lowest grade gold is that 

 of veins in the Tertiary lavas, much of it reaching electrum proportions 

 of silver. In general the gold in the auriferous quartz-sulphide veins 

 in granite is of higher grade, but that in quartz veins free from large 

 amounts of sulphides and inclosed in slates or other metamorphic 

 rocks is of still better grade. Gold in oxidized ore is commonly purer 

 than that in unaltered primary ore, especially if the latter be sulphidic. 

 This is due to the tendency of the silver to be selectively extracted 

 by percolating meteoric waters. As an instance of this may be men- 

 tioned the observation of Umpleby " that whereas the gold of the 

 primary ore of the Golden Sunbeam mine, Yankee Fork district, 

 Custer County, is electrum, in the oxidized zone native gold and native 

 silver occur in relatively pure form, often side by side, the two metals 



10 Robert N Bell. Annual Report of State Inspector of Mines on the Mining Industry of Idaho for 1906 , 

 p. 115. 

 » Joseph B. Umpleby. U. S. Geol. Survey Bull. 539, p. 88, 1913. 



