Note 50 — Chapter XVII · CONSERVATION OF MINERAL RESOURCES
Reading edition
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.
Reading edition
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.