@misc{indiciae29f8ff701a7f,
  title = {Geologic map of the North Keystone mine, Copperopolis, California},
  author = {G.R. Heyl and J. B. Hadley and G.L. Quick},
  year = {1943},
  doi = {10.3133/ofr4341},
  url = {https://pubs.usgs.gov/publication/ofr4341},
  note = {Source identifier: ofr4341}
}