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.

Loc. cit.

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.

Loc. cit. p. 12.

Smith, George Otis, and Lesher, C. E., The cost of coal: Science, vol. 44, 1916, p. 768.

Loc cit., pp. 768-769.

Loc. cit., p. 771.

Van Hise, Charles R., Coöperation in industry, pp. 7-8, Address given before annual meeting of the National Lumber Manufacturers' Association, Chicago, Illinois, May 31, 1916.