Hoover, Herbert C., Principles of mining: McGraw-Hill Book Co., New York, 1909, p. 32.
Leith, C. K., Use of geology in iron ore exploration: Econ. Geol., vol. 7, 1912, pp. 662-675.
Van Hise, C. R., and Leith, C. K., Geology of the Lake Superior region: Mon. 52, U.S. Geol. Survey, 1911.
CONSERVATION OF MINERAL RESOURCES
Campbell, M. R., The coal fields of the United States: Prof. Paper 100-A, U. S. Geol. Survey, 1917, p. 24.
Final report of the Royal Commission on coal supplies: House of Commons, London, vol. 16, 1905.
Report of the National Conservation Commission: Senate Document No. 676, 60th Congress, 2d session, Govt. Printing Office, Washington, 1909.
Van Hise, C. R., The conservation of natural resources in the United States: Macmillan Co., New York, 1910.
Haas, Frank, The conservation of coal through the employment of better methods of mining: Abstract of paper presented to Pan-American Scientific Congress, Washington, Dec., 1915-Jan., 1916.
Adams, Frank D., Our mineral resources and the problem of their proper conservation: 6th Ann. Rept., Commission of Conservation, Canada, 1915, pp. 52-69.