Approximately 800 square miles of arid Basin and Range type terrain was mapped and described during this geologic study. A diverse sequence of Paleozoic and Mesozoic pre-batholithic strata was recognized and differentiated where possible. The pre-batholithic sediments were intruded by at least two periods of batholithic emplacement, with the later being the mid-Cretaceous emplacement of the Peninsular Range Batholith, which was the more extensive of the two. Post-batholithic strata range in age from Paleocene (?) to Recent. Paleocene (?) strata are only locally exposed and consist of clastic deposits derived from local granitic and metamorphic terrains, and a single andesite flow dated by the potassium-argon method. Eocene conglomerate at a single locality in the Sierra San Felipe-Sierra Santa Rosa basin contains clasts that resemble Eocene conglomerates and gravels from the Sierra Juarez to the west. Sedimentary rocks of volcanic derivation (post-Eocene conglomerate) may be of Oligocene age as they predate the oldest Miocene volcanics in the northwestern part of the study area. The Miocene Epoch has the most diverse representation of strata. Basic, intermediate, and acidic volcanic flows and volcanically derived clastic deposits from the bulk of the Miocene strata; however, a sequence of marine strata of late Miocene age are exposed in the south-central part of the study area. The Pliocene Epoch is represented by a widespread fanglomerate unit, locally intertongued with marine deposits containing locally well-preserved molluscan faunas. Various Quaternary units were also differentiated. The Quaternary deposits include fanglomerate, marine strata, older alluvium, beach ridges, lacustrine strata, dune sands, and alluvium. Four periods of faulting were recognized as occurring in the Paleocene (?), late Miocene, late Pliocene, and Quaternary. Three periods of folding were determined to be Eocene to pre-Miocene volcanics, late Miocene, and late Pliocene to early Pleistocene (?). In conclusion, geologic evidence indicates the regional paleo-slope was generally westward through the Eocene Epoch. Extensive vulcanism and tensional faulting during the Miocene Epoch suggests the beginning of dilation of the Gulf of California. The sedimentary evidence of eastward transportation of coarse clastic sediments indicates that depression accompanied dilation. During the late Miocene (ten to seven million years ago) the Gulf of California opened to the south with normal marine circulation as evidenced by fossiliferous marine strata.