The Chiricahua and Dos Cabezas mountains are typical of the Basin-and-Range Province in southeast Arizona. Well exposed Paleozoic and Mesozoic marine strata overlie a Precambrian basement complex of schist and granite. The 7,600 feet of Paleozoic strata were deposited on a slowly subsiding shelf, beginning in middle(?) or late Cambrian time. Sedimentation was interrupted by an epeirogenic uplift that caused the widespread unconformity between Lower Ordovician and Upper Devonian strata. The pronounced angular discordance between the Paleozoic strata and the Lower Cretaceous beds was caused by a pre-Cretaceous orogeny. Compressive stresses during the post-Comanche--pre-Pliocene orogeny strongly deformed all the strata. The range was uplifted later in the Cenozoic by high-an le faulting.

The oldest formation is the Bolsa quartzite (Middle(?) to Upper Cambrian), which unconformably overlies the basement rocks. It is arkosic to quartzose and ranges from 320 to 600 feet in thickness. The El Paso formation (Upper Cambrian-Lower Ordovician) conformably overlies the Bolsa and ranges in thickness from 340 to 715 feet. It consists of dolomite and limestone. The overlying Portal formation (Upper Devonian) is here named for exposures 2½ miles northwest of the village of Portal. The formation ranges from 200 to 342 feet in thickness and consists of the following four members, in ascending order: (1) alternating limestone and shale, (2) black siliceous shale, (3) alternating limestone and shale, (4) bioclastic limestone. The Escabrosa limestone (Lower Mississippian) consists of cherty crinoidal limestone ranging from 630 to 730 feet in thickness. It is overlain by the Paradise formation (Upper Mississippian) which consists of about 150 feet of alternating thin limestone and clastic strata.

The Pennsylvanian and Permian systems are represented by the Naco group, composed of about 5,500 feet of limestone and clastic strata. The Naco group is subdivided into the following five formations, in ascending order: Horquilla, Earp, Colina, Scherrer, and Concha. The Horquilla is dominantly limestone and ranges in age from the Morrow to Missouri series. It is conformably overlain by the Earp formation (Missouri to Wolfcamp age) which consists of alternating limestone, sandstone, and shale. The Colina (Wolfcamp to Leonard age) is aphanitic black limestone. The Scherrer (Leonard age) consists of quartzitic sandstone with a basal red siltstone member. The uppermost formation is the Concha limestone (Leonard to Guadalupe? age) which is very fossiliferous and cherty. The dominantly carb nate Naco sequence of southeast Arizona is equivalent to the thinner, dominantly clastic Pennsylvanian-Permian strata of the Colorado Plateau.

The Mesozoic era is represented by the Bisbee group of Lower Cretaceous clastic strata. The Glance conglomerate at the base reaches a maximum thickness in excess of 1,000 feet. The overlying 2,500 feet of the Bisbee group is divided into three lithologic units, but no formal names are proposed. The lowest unit is a red siltstone. This is overlain by the thin middle limestone unit, which is probably equivalent to the Mural limestone at Bisbee. The uppermost unit consists of alternating quartzite and siltstone.