@misc{indiciae439b6a2ccc05,
  title = {A definition of coal},
  author = {S. I. Tomkeieff},
  year = {1957},
  doi = {10.2113/gsecongeo.52.5.584},
  url = {https://pubs.usgs.gov/publication/70213491},
  note = {Source identifier: 70213491}
}