COAL FIELDS OF THE UNITED STATES
Kinds of Coal
The United States Geological Survey classifies coals, first of all, according to “rank,” depending upon both chemical and physical characteristics. Anthracite, which contains the largest percentage of carbon, ranks highest, and lignite, with the smallest percentage of carbon, lowest. Coals of the same rank are said to be of high or low “grade,” according to whether they contain a relatively small or large percentage, respectively, of ash and sulphur. The ranks recognized by the Survey are: Anthracite, semi-anthracite, semi-bituminous, bituminous, sub-bituminous, and lignite.
Courtesy of U. S. Bureau of Mines
AN “ENTRY” IN A COAL MINE
Showing timbered roof
Courtesy of U. S. Bureau of Mines
COATING WALLS OF A MINE WITH CEMENT