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176
MINERAL RESOURCES.
against 2,586 in 1907. The deep mines number 2,864 in 1908, against 2,604 in 1907. In Alaska it has proved difficult to record all the small operations, and the number of placer mines is therefore, in part, an estimate.
There are no great changes in the number and distribution of placer mines. The number of deep mines or lode mines has increased considerably during the last three years from 2,114 in 1906 to 2,864 in 1908. The number of producing deep mines decreased in 1908 in Arizona, South Dakota, Wyoming, and Michigan; the change in Michigan was, however, simply due to the tracing of the silver product to a limited number of mines. Notable increases in the number of
p roducing lode mines are recorded from Colorado, Idaho, Montana, evada, and Utah. In Nevada the deep mines increased from 216 in 1907 to 393 in 1908.
The best guide to the development of deep mines is, however, found in the record of tonnage of ores mined, which is given in a subsequent table.
A proper classification of the western ores is very difficult on account of their complex nature and the transitions between the various classes, however they may be defined. The classification in the fol­lowing table is based on the qualrty of the ore as mined. The ores are divided into dry or siliceous ores, which comprise the gold and silver ores proper; further into copper ores, lead ores, zinc ores, copper-lead-zinc ores, and lead-zinc ores. It is not possible to divide the dry or siliceous ores into gold ores and silver ores, for the two metals are present in them in all proportions. However, the silver ores mined exclusively for silver form a very small class and less than 100,000 tons of them were mined. Under copper ores are classed those which contain 2-1/2 per cent or more of copper, as well as those which contain less than this amount, provided their principal value is in copper. The Lake Superior ores contain, for instance, only from 1 to 2 per cent of copper, and from the Bingham porphyry ores less than 1.5 per cent is recovered. The lead ores are those which con­tain over 4-1/2 per cent of lead, irrespective of their tenor in precious metals. Special divisions of mixed ores have been established. A comparatively small amount of zinc is mined as zinc ores, and these contain at least 25 per cent of that metal. Most of the argentiferous zinc ores of the Rocky Mountain States are mined as lead-zinc ores and are subsequently concentrated.