Note 42 — Chapter XVII · CONSERVATION OF MINERAL RESOURCES
Reading edition
Campbell, M. R., The coal fields of the United States: Prof. Paper 100-A, U. S. Geol. Survey, 1917, p. 24.
Reading edition
Campbell, M. R., The coal fields of the United States: Prof. Paper 100-A, U. S. Geol. Survey, 1917, p. 24.