Bocks ranging in age from Precambrian to Recent crop out in the Pima district. Granite, which locally contains many large inclusions of schist and hornfels, is believed to be Precambrian, as it appears to be overlain unconformably by the Bolsa quartzite of Middle Cambrian age. Probably all the Paleozoic formations of the region are represented, but only the Bolsa quartzite, at the base of the Paleozoic section, has been differentiated on the geologic map. Overlying the Paleozoic rocks is a complex of sedimentary and volcanic rocks assigned to the Cretaceous(?). Sedimentary units include conglomerate, arkose, graywacke, quartzite, shale, and a few thin beds of limestone. Volcanic units Include andesitic and rhyolitic types. The stratigraphic sequence within the Cretaceous (?) complex is not known. Preliminary subdivisions based on lithology are shown on the map. Diorite, andesite, granodiorite, and quartz monzonite porphyry postdate the Cretaceous (?) complex and are assigned to the Late Cretaceous or early Tertiary. These rocks, with the possible exception of some of the diorite and andesite, were intruded after a major orogeny that affected the Cretaceous (?) complex. They are all mineralized locally. The quartz monzonite porphyry, which is probably the youngest of these rocks, is associated in space with several large copper deposits, suggesting that porphyry and ore are related genetically. In one part of the district, folded and faulted Cretaceous (?) rocks are unconformably overlain by a welded rhyolitic tuff. It is not clear whether the tuff is older or younger than the ore. A deformed postmineralization formation, here named the Helmet fanglomerate, overlies the ryholitic tuff unconformably. The fanglomerate consists predominantly of coarse, ill-sorted and ill-bedded conglomerate characterized by angular pebbles, cobbles, and boulders in an abundant silty matrix. Intercalated are lava flows of porphyritic andesite, scarce thin beds of rhyolitic tuff and tuffaceous sediments, and lentils and tongues of monolithologic breccia which appear to have been emplaced as landslides. Several stratigraphic units are indicated by variations in color, composition of fragments, sedimentation features, and .presence or absence of tuffaceous material. The formation is probably at least 10,000 feet thick. It is believed to be made up of alluvial-fan deposits derived from the west, southwest, or northwest No fossils have been found, but tentative correlations suggest that the fanglomerate may be of early Miocene age. Andesite dikes cut the Helmet fanglomerate; and Quaternary alluvial deposits cover much of the central and eastern part of the area described. The structural relations of the Helmet fanglomerate indicate two orogenies that postdate the Cretaceous (?) rocks. The earlier, pregranodiorite and premineralization, resulted in complex fold and fault structural features that trend 64 CONTRIBUTIONS TO ECONOMIC GEOLOGY Jiorthwest. The later, post-mineralization and post-Helmet, resulted in a large thrust fault and steep tilting to the southeast. Rotational effects of the post-Helmet orogeny must be removed to restore the structure at the time of mineralization. The San Xavier thrust fault beneath the northeastern part of the district cuts the Helmet fanglomerate and is therefore a postmineralization thrust. Small faults in boulders of the fanglomerate in the thrust plate suggest that the plate moved north-northwest. Similarities in the geology of the plate and of the autochthonous block to the south suggest that the plate moved about 6% miles north-northwest.