The Mountain City Formation was named by Granger and others (1957) for exposures at the Mountain City Mine. The type locality was designated at the crest of a hill north of Mill Creek in Sec. 2, T. 45 N., R. 53 E, in the Mountain City Quadrangle by Coats (1969).

The Mountain City Formation conformably overlies the Late Mississippian Nelson Formation and is overthrust by the Reservation Hill Formation. Its age is questionable and may be Mississippian or Early Pennsylvanian (Coats, 1969). It is considered a metamorphosed equivalent of the Chainman Formation (Coats, 1985).

At the type section in the Mountain City Quadrangle, the Mountain City Formation is composed of fine-grained, dark-gray to black, siliceous schist. The schist has a quartz-rich matrix and contains minor amounts of orthoclase, sericite, and calcite, as well as very-fine carbonaceous material (Coats, 1969). A few limey beds are also present in the formation and contain tremolite needles as a result of contact metamorphism along intrusives. The section in the Owyhee Quadrangle is essentially identical, with the addition of a few thin beds of meta-rhyolite tuff which are now quartz-plagioclase-orthoclase hornfels (Coats, 1971).

The Mountain City Formation is at least 4,000 feet thick in the type locality in the Mountain City Quadrangle (Coats, 1969).

The Mountain City Formation is exposed west of the Owyhee River from the Mountain City Mine to Fawn Creek in the Mountain City and Owyhee Quadrangles.

The Mountain City Formation is most probably a strongly contact metamorphosed section of Chainman Formation. The reader is referred to the depositional setting description for the Chainman Formation.