Comparison
of the figures for 1915 shows a net total increase of 344 producing
mines following decreases in 1912, 1913, and 1914. The total number of
properties operated in 1915 was 52 in excess of the average for the
last 10 years. The number of placer properties worked in 1915 showed a
decrease of 95 and was nearly 600 less than the number operated in
1906. This is a natural result due to the exhaustion of the rich
gravels and to the acquisition of large areas of placer ground by
dredge mining companies. The number of deep mines operated increased
from 3,158 in 1914 to 3,597 in 1915, or 454 more than the average for
10 years. High prices for copper, lead, and zinc in 1915 resulted in
more deep mines being operated than in any year for which data are
available by the Survey.
In
total number of operating mines Colorado again ranked first with 821,
followed in order by Nevada with 799, Alaska with 741, Montana with
643, California with 608, Arizona with 517, and Idaho with 412. In
number of placers Alaska led with 700, followed by California with 287,
and Idaho with 186; and in deep mines Colorado led with 786, followed
by Nevada with 696, Montana with 482, Arizona with 468, and California
with 321.
ORE PRODUCTION, CLASSIFICATION, AND AVERAGE VALUE.
The
best index of deep mining is, of course, the quantity, metallic
content, and value of ore mined, rather than the number of mines or of
operators. In the next following table is given the production of ore
by classes of ore and by States and Territories from mines producing
gold and silver and the average extraction value of precious metals per
ton.
The
classification 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 very small. The copper ores
include those containing over 2-1/2 per cent of copper, or less than
this percentage in the cases of the great disseminated copper deposits
of the West and of the Lake Superior ores; in general, 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; but ores of lower grades in lead and
