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 December 6, 2016.
Note: Inconsistencies in capitalization and punctuation within the Digital Library may affect this citation. Before using this citation in formal writing, please consult your preferred style manual. More information.