Pennsylvanian and Permian rocks in an area of approximately 240,000 square miles in the Cordilleran area have numerous features in common which justify their consideration together. For this study they are treated in the light of nomenclature, thickness, stratigraphic relationships, correlation, conditions of deposition, and relative significance. Approximately 50 formal names can be applied to units of formation rank in these systems. The sediments were deposited in well-defined basins and troughs, and upon banks adjacent to positive areas within the western part or eugeosyncline, and eastern part or miogeosyncline. During Permian time at least, a volcanic archipelago probably existed west of the eugeosyncline; the latter geosyncline has volcanic and associated materials The miogeosyncline received mostly marine clastic sediments and various carbonates. Manhattan line separated the two geosynclines, at times only as an arbitrary boundary, and at other times as an epeirogenic to orogenic belt. East of the miogeosyncline was a well-defined platform or shelf which varied in time and space in proportion between the continental craton east of it and the geosyncline to the west.

Individual formations within the Cordilleran area range in thickness from a few hundred feet to some of thousands of feet (26,000 for the Oquirrh formation). Many of these formations straddle systemic boundaries; many are time-rock units. Sedimentation was not everywhere constant nor uninterrupted for the duration of these periods of time. In fact, epeirogeny and orogeny were characteristic (but localized at times). Antler orogeny in Devonian and late Mississippian to early Pennsylvanian time, however, affected much of Nevada and western Idaho on a profound scale. Certain positive areas, such as one in the general region of western Utah and eastern Nevada, were tectonically active during parts of Pennsylvanian time, and late in the period existed as important land masses. Basins and t oughs adjacent to this particular positive area, such as the one between it and the Manhattan area to the west, subsided with acceleration at times. Such prominent depocenters as the Oquirrh basin, Wood River basin, Wells basin, Diamond Peak and Ely basin, Bird Spring basin, and others, were dominantly negative parts of this area, and were integral parts of the larger miogeosyncline. Most of these, along with counterparts in the eugeosyncline farther west, persisted as subsiding repositories throughout most of Permian time also. Orogeny in mid-Permian affected much of the eugeosyncline belt on a grand scale.

The Las Vegas line of southern Nevada acted as an important Paleozoic hinge line, separating shelf facies to the south and southeast from basin facies on the west and northwest, and was particularly effective in the Pennsylvanian and Permian. It is the southern counterpart of the Wasatch line.

Reef complexes composed of bioherms, biostromes, and bioclastic beds are abundant and widespread in both Pennsylvanian and Permian rocks. Many of these sediments have strong hydrocarbon odors and may, therefore, be potential sources and reservoirs of petroleum. Drilling for oil and gas in which these late Paleozoic rocks have been tested is still in no more than preliminary state for most of the Cordilleran area.