Thickness of Bituminous-Coal and Lignite Seams at All Mines and Thickness of Overburden at Strip Mines in the United States in 1950, report, September 1952; (https://digital.library.unt.edu/ark:/67531/metadc67062/m1/21/: accessed May 3, 2024), University of North Texas Libraries, UNT Digital Library, digital.library.unt.edu; .