@misc{indiciae75407c9a9da8,
  title = {Copper-Zinc deposits of the Penn Mine Calaveras County, California},
  author = {George Richard Heyl and M.W. Cox and J. H. Eric},
  year = {1945},
  doi = {10.3133/ofr4551},
  url = {https://pubs.usgs.gov/publication/ofr4551},
  note = {Source identifier: ofr4551}
}