Mining experiment station in Colorado, hearings, Dec. 18, 1911 [Jan. 11, 1912, on H. 11414, 11853, and 15784, relating to establishment of mining experiment stations at Silverton, Colo., and at Salt Lake City, Utah, and to amend act to establish in Department of Interior Bureau of Mines, so as to extend jurisdiction and operations of Bureau of Mines to slate, granite, and cement-rock quarries]