This map is part of a folio of maps of the Ajo and Lukeville 1 ° x 2° quadrangles, Arizona, prepared under the Conterminous United States Mineral Assessment Program. Other publications in this folio include U.S. Geological Survey Miscellaneous Field Studies Maps MF-1831-A and MF-1831-B and Open-File Reports 82-119, 82–599, and 83–734. Open-File Reports 82-119 and 83–731 constitute the basic data and initial interpretation on which this discussion is predicated.