The 1,400 square kilometer study area is located at the geographic center of the Baja California Peninsula and contains a disseminated porphyry copper deposit of unusual age and genesis. Cretaceous eugeosynclinal rocks, probably partly correlative with the Albian-Aptian Alisitos Formation, were deformed and intruded by plutonic intrusions between 120 and 100 my (million years before present time) B. P. The compositions of the intrusive bodies range from ultrabasic to granodiorite with the average near the intersections of the compositional fields of granodiorite-tonalite, quartz diorite, and quartz monzodiorite. Contact metamorphism of a belt of coextensive prebatholithic strata, now exposed along the western margin of the batholith, produced subparallel facies zones. These facies zones, produced in the low-grade metamorphic margin of the batholith, increase in rank from nonmetamorphic in the west through albiteepidote hornfels facies to hornblende-plagioclase hornfels facies in the east. Coincident with the metamorphism, hydrothermal activity derived from mineral dehydration reactions, connate water and outgassing plutonic rocks formed vein-type ore deposits of copper, iron and gold in the albite-epidote hornfels facies zone. The El Arco diorite porphyry, a small stock of the 129 batholith, intruded along the fractured axial plane of an anticline in the prebatholithic strata . The additional heat provided by the porphyry, and the porosity of the fractured rocks along the axial plane may have controlled the metasomatism and ore mineralization of the deposit. The mineralization probably occurred between 107 my and 93 my B. P. Rapid cooling of the terrane, due to uplift and erosion, occurred at the rate of about 50° C per million years possibly quenching further hydrothermal activity in the low-grade metamorphic margin. The batholith was unroofed to near it s present level by the end of the Cretaceous Period. In early Cenozoic time peneplained basement rocks were overlain by a thin veneer of westward dipping (marine ?) sedimentary rocks of the Paleocene- Eocene series. Erosion exposed much of the crystalline basement rocks in the later part of the Eocene and the Early Oligocene. During Late Oligocene-Miocene time older rocks were covered by tuffaceous sandstone. These strata are primarily continental deposits. They increase in proportion of rhyolitic tuff materials to the east, the probable source direction. Extrusive volcanic rocks of Latest Miocene and Pliocene age covered the surface of the Miocene series, The lava flows, generally basalt and andesitic basalt, covered rocks of all ages. Erosion exhumed significant portions of the crystalline batholith and prebatholithic rocks during Upper Pliocene, Pleistocene and Recent time leaving the Pliocene volcanic rocks as low, isolated mesas protecting the underlying rocks.