@misc{indiciae1bbe02c50b8c,
  title = {Preliminary geologic map of Puerto Rico and adjacent islands},
  author = {Reginald Peter Briggs},
  year = {1961},
  doi = {10.3133/ofr6124},
  url = {https://pubs.usgs.gov/publication/ofr6124},
  note = {Source identifier: ofr6124}
}