A readily combustible rock containing more than 50% by weight and more than 70% by volume carbonaceous material, formed by compaction and induration of variously altered plant remains...
This category is also used for anthracite and bituminous.

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Coal fields of the conterminous United States - National Coal Resource Assessment updated version

This map sheet with accompanying Geographic Information System (GIS) project is an update of the existing U.S. Geological Survey (USGS) Conterminous U.S. Coal Fields map compiled using data primarily from the USGS National Coal Resource Assessment (NCRA).