TY  - RPRT
TI  - Coal resource classification system of the U.S. Geological Survey
AU  - Gordon H. Wood
AU  - Thomas M. Kehn
AU  - M. Devereux Carter
AU  - William C. Culbertson
PY  - 1983
DO  - 10.3133/cir891
UR  - https://pubs.usgs.gov/publication/cir891
ID  - cir891
ER  -