The
total number of properties operated in 1920 was 1,652 less than the
average for the last 15 years. The number of placer properties worked
in 1920 showed a decrease of 93, and the total was 1,304 less than the
number operated in 1906. This is a natural result of the exhaustion of
the rich gravels and of the acquisition of large areas of placer ground
by dredge mining companies. The number of deep mines operated was 809
less than the average for 15 years. Owing to the high cost of labor and
material in 1920 fewer deep mines were operated than in any other year
since 1906.
ORE PRODUCTION, CLASSIFICATION, AND AVERAGE VALUE.
The
best index of deep mining is, of course, found in the quantity,
metallic content, and value of ore mined, rather than in the number of
mines or of operators. In the next following table are given the
production of ore from mines producing gold and silver and the average
extraction value of precious metals per ton.
The
classification adopted is necessarily arbitrary in part. The complex
nature of western ores, especially, and the gradations from one
well-recognized class to another render essential a fixed termi-
