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
folÂlowing 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 conÂtain 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.
