Não informado. (1931). International coal trade. U.S. Department of the Interior, Bureau of Mines.
Chicago-referens (17:e uppl.)Não informado. International Coal Trade. Washington: U.S. Department of the Interior, Bureau of Mines, 1931.
MLA-referens (8:e uppl.)Não informado. International Coal Trade. U.S. Department of the Interior, Bureau of Mines, 1931.
Varning: dessa hänvisningar är inte alltid fullständigt riktiga.