@misc{indiciae31b651083b5c,
  title = {Coal resource classification system of the U.S. Bureau of Mines and U.S. Geological Survey},
  author = {Members of the Engineering Geology and Ground Water Branches, U.S. Geological Survey},
  year = {1976},
  doi = {10.3133/b1450b},
  url = {https://pubs.usgs.gov/publication/b1450B},
  note = {Source identifier: b1450B}
}