TY  - RPRT
TI  - The coal fields of the United States
AU  - M. R. Campbell
AU  - J. A. Bownocker
AU  - F. R. Clark
PY  - 1929
DO  - 10.3133/pp100
UR  - https://pubs.usgs.gov/publication/pp100
ID  - pp100
ER  -