Young, W. H. & Anderson, R. L. Thickness of Bituminous-Coal and Lignite Seams at All Mines and Thickness of Overburden at Strip Mines in the United States in 1950. Washington D.C.. UNT Digital Library. http://digital.library.unt.edu/ark:/67531/metadc67062/. Accessed April 16, 2014.
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