@misc{principles+of+mining+valuation+organization+and+administration+copper+gold+lead+silver+tin+and+zinc,
    author = {Hoover, Herbert},
    title = {Principles of mining : valuation, organization and administration : copper, gold, lead, silver, tin and zinc},
    address = {New York i London},
    publisher = {McGraw-Hill Book Company, Inc.},
    date = {1909}
}