@misc{indiciae7e072de82eed,
  title = {Marking geological specimens},
  author = {Earl T. Apfel},
  year = {1929},
  doi = {10.1126/science.70.1815.358.a},
  url = {https://pubs.usgs.gov/publication/70206623},
  note = {Source identifier: 70206623}
}