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248                                 MINERAL RESOURCES, 1912.
little change; but in Washington the number of producing placers fell from 24 to 12 and the deep mines from 36 to 34. In the Central States there are no placers and the small number of deep mines pro­ducing precious metals varies little from year to year. In the East­ern States the numbers of producing placers and deep mines were the same in 1912 as in 1911.
In total number of producing mines California led with 1,041, followed by Colorado with 889, Alaska with 752, Nevada with 704, Montana with 604, Arizona with 445, and Idaho with 408. The order was the same in 1911, except that Montana preceded Nevada.
ORE PRODUCTION, CLASSIFICATION, AND AVERAGE VALUES.
The best index of deep mining is, of course, the tonnage, content, and value of ore mined, rather than the number of mines or of opera­tors. In the next following table is given the ore production, by classes of ore and by States and Territories, of mines producing gold and silver and the average extraction value of precious metals per ton.
The clasification adopted is necessarily arbitrary in part. An ore is generally understood to be worked at a profit for one or more metals. The complex nature of western ores, especially, and the gradations from one well-recognized class to another render essential some fixed measures for the terminology used. The dry or siliceous ores com­prise gold and silver ores proper, as well as fluxing ores carrying con­siderable quantities of iron and manganese oxides and very small quantities of gold and silver, and precious metal bearing ores carrying copper, lead, or zinc in quantities too low to classify them as copper, lead, zinc, or mixed ores. The distinction between gold and silver ores is not here made. The total number of silver mines and the total production of true silver ores are both relatively very small. The copper ores include those containing over 2-1/2 per cent of copper, or less than this in the cases of the great disseminated copper deposits of the West and of the Lake Superior ores; the lead ores are those con­taining 4-1/2 per cent or more of lead, and the zinc ores are those contain­ing 25 per cent or more of zinc, both irrespective of their precious metal content. The mixed ores are combinations of the ones enumerated.