"Revolution in coal. "; "Wolfert, Ira."; "Coal mines and mining--Pennsylvania.", "Coal mines and mining--West Virginia.", "Coal mines and mining--United States."; "Wolfert, Ira."; "1954"; "Journal article reprint from: Reader's Digest, 1954:Dec. "; ""; "P3409"; "pec"; "Coal mines and mining--West Virginia.", "Coal mines and mining--United States.", "Coal mines and mining--Pennsylvania."; "14256"; .