[44] Report of the National Conservation Commission: Senate Document No. 676, 60th Congress, 2d session, Govt. Printing Office, Washington, 1909.
[45] Van Hise, C. R., The conservation of natural resources in the United States: Macmillan Co., New York, 1910.
[46] 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.
[47] Adams, Frank D., Our mineral resources and the problem of their proper conservation: 6th Ann. Rept., Commission of Conservation, Canada, 1915, pp. 52-69.
[48] Chance, H. M., Address before the mine engineering class of the Pennsylvania State College, Quoted by F. W. Gray, The conservation of coal: Bull. 47, Can. Mining Inst., 1916, p. 201.
[49] Loc. cit.
[50] Campbell, M. R., and Parker, E. W., Coal fields of the United States, Papers on the conservation of mineral resources: Bull. 394, U. S. Geol. Survey, 1909, p. 12.
[51] Loc. cit. p. 12.
[52] Smith, George Otis, and Lesher, C. E., The cost of coal: Science, vol. 44, 1916, p. 768.
[53] Loc cit., pp. 768-769.