TY  - RPRT
TI  - Coal resource classification system of the U.S. Bureau of Mines and U.S. Geological Survey
AU  - Members of the Engineering Geology and Ground Water Branches, U.S. Geological Survey
PY  - 1976
DO  - 10.3133/b1450b
UR  - https://pubs.usgs.gov/publication/b1450B
ID  - b1450B
ER  -