The terrain drained by the Casas Grandes River and its tributaries in northwestern Chihuahua, Mexico, consists entirely of recent volcanics such as rhyolites, andesites, basalts and tuffs. The climate is arid or semi-arid. Thin sections made from rock specimens as well as river transported sediments were used to determine the characteristics and derivation of the minerals, such as feldspars and less stable ferromagnesians, are well represented in rocks from which thin sections were made but they are rarely present in the detritals. Olivine as shown in thin sections has undergone various stages of alteration which may, in part, account for the abundance of magnetite. Detrital minerals in order of abundance are quartz or chalcedony, magnetite, olivine, hypersthene, andesite, basaltic hornblende, tourmaline, mica, hematite, zircon and apatite.