Não informado. (1931). International coal trade. U.S. Department of the Interior, Bureau of Mines.
Lua i Stíl Chicago (17ú heag.)Não informado. International Coal Trade. Washington: U.S. Department of the Interior, Bureau of Mines, 1931.
Lua MLA (8ú heag.)Não informado. International Coal Trade. U.S. Department of the Interior, Bureau of Mines, 1931.
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