Central Idaho contains a thick assemblage of Paleozoic strata, mostly in the region extending from the eastern border of the Idaho batholith in the Sawtooth Mountains northeastward to Montana, and from the Snake River Plain N. to about 45 degrees N. The Milligen Formation is the oldest generally recognized Carboniferous formation and is of Mississippian age. It is overlain by the Wood River Formation, at least 8000 ft. thick, composed dominantly of siliciclastic rocks; near Bellevue the lower third is of Pennsylvanian age whereas the upper part is of Permian age. The term "Brazer limestone" should be dropped for central Idaho outcrops, and the unit is here named the White Knob limestone. It is more than 7300 ft. thick at the type locality in the White Knob Mountains and consists mainly of pure limestone, commonly with chert, and locally some minor nonpersistent silty, sandy, and even conglomeratic beds. The White Knob limestone ranges in age from Early Mississippian to Early Permian. In the Mackay quadrangle, a stratigraphic unit here named the Copper Basin formation, is probably more than 10,000 ft. thick, is composed of varying proportions of silt-stone, sandy quartzite, conglomerate, and limestone, and interleaves with lithic equivalents of the Milligen, Wood River, and White Knob formations. The Copper Basin formation is probably overlain locally by the Wood River Formation and White Knob limestone and may be underlain in part by the Milligen Formation. All diagnostic fossils found from the formation are of Late Mississippian age,but its lateral interfingering with the Milligen, Wood River, and White Knob formations suggest an age range from Early Mississippian to Early Permian.