The accompanying geologic maps depict the general geology of a two-mile wide strip centered about the Salt River, starting just below Roosevelt Dam and extending westward 36 miles to just below Granite Reef Dam. The geology was accomplished from October 1980 through April 1981, with the aid of about 25 hours of helicopter logistical support time. Discussions and/or field assistance by Mike Pryor, Joe Bailey, Dick Raymond, Paul Damon, and Tom McGarvin are greatly appreciated. Geologic Summary The lower Salt River below Roosevelt Dam has cut a gorge which exposes much of the geology of the northern Superstition volcanic field. The silicic field of late Oligocene (?) and Miocene age is one of the half dozen or so found in southern and western Arizona. The volcanics extend continuously from around Apache Lake Resort westward to Stewart Mountain Dam, and discontinuously farther west to the Mt. McDowell area, around Granite Reef Dam. The Superstition volcanics were deposited on Precambrian crystalline rocks, dominated by a biotite quartz monzonite which age dates at about 1400 m.y., and which is probably equivalent to the Ruin and Oracle granites further east and south. The northern Superstition volcanics are dominated by silicic pryoclastics, plugs, ash flows, agglomerates, and late stage intrusive domes. Rocks of andesitic composition are subordinate in volume although important in the eastern part of the area.