@misc{indiciae7a7b04fbf909,
  title = {The coal fields of the United States},
  author = {M. R. Campbell and J. A. Bownocker and F. R. Clark},
  year = {1929},
  doi = {10.3133/pp100},
  url = {https://pubs.usgs.gov/publication/pp100},
  note = {Source identifier: pp100}
}