Underground workings, American Girl Mine, Cargo Muchacho Mining District, Imperial County, California: Supplement 3 from "A Tertiary mammalian fauna from the San Antonio Mountains near Tonopah, Nevada. Geology and mineral deposits of the Cargo Muchacho Mountains, Imperial County, California" (Thesis)

Part I: A Tertiary mammalian fauna from the San Antonio Mountains near Tonopah, Nevada: A collection of mammalian fossils from near Tonopah, Nevada, contains nineteen pieces, six of which have been reported from no other locality. With more than 225 individuals in the collection, the assemblage approaches a true representation of the numerical frequency of species in a plains fauna. Abundance of skeletal elements of Hypohippus and Merychippus affords material for detailed osteological comparison. Geological reconnaissance in the area about California Institute of Technology Vertebrate Paleontology Locality No. 172 led to the discovery of a contemporary inland lake faunal facies containing diatoms, molluscs and fish. The occurrence of the mammalian fossils in gravels and cherts, as well as in clays, indicates that the bones were deposited near the entrance of a stream into a saline lake. The small stratigraphic extent of the quarry, the close association of the bones, and the limited morphological variation within abundantly represented species suggests that the fauna is unmixed. The mammalian assemblage is correlated with the uppermost Barstow and is designated as Upper Miocene in age. Part II: Geology and mineral deposits of the Cargo Muchacho Mountains, Imperial County, California: The study of the Cargo Muchacho Mountains presents the first account of the geology of southeasternmost California. In these mountains pre-Mesozoic kyanite schist and arkosite are intruded by a series of granitoid rocks probably Mesozoic in age. The two youngest rocks of this series were formed by granitization of arkosite. Andesite dikes of minor importance intruded the area probably in Tertiary time. Remnants of a once extensive flow of olivens basalt cover a few outlying hills. Alluvial deposits of three distinct ages occur in the area. Two of these, which are pre-basalt in age, offer evidence that the Colorado River formerly flowed through the region. The Cargo Muchacho Mountains constitute an important gold mining district. The occurrence of the ore is shown to have been controlled by the regional structure. The ore deposits are mesothermal and are locally characterized by extreme alteration of the wall rock. Kyanite and sericite are also mined in the district. The record of mining activity and production of the district is brought up to date