@misc{indiciaed9cd035a318b,
  title = {Analyses of coal samples from various parts of the United States},
  author = {M. R. Campbell and F. R. Clark},
  year = {1916},
  doi = {10.3133/b621p},
  url = {https://pubs.usgs.gov/publication/b621P},
  note = {Source identifier: b621P}
}