The Geological Survey has made a reconnaissance of springs in the Mogollon Rim region in central Arizona. This region is the source of much of the water in the Gila, Salt, and Verde Rivers. The region has not previously been systematically studied with respect to the occurrence of ground water. The Mogollon Rim is an escarpment that extends about 200 miles in a northwest direction from near Clifton and Morenci in southeastern Arizona and gradually disappears north of Prescott. Lumbering, ranching, and in local areas copper mining are the principal industries. Main lines of drainage extend north on the plateau, north of the rim, and south or southwest below the rim. For convenience in discussion and because of structural differences, the region has been separated into western, central, and eastern divisions. Pre-Cambrian to Recent rocks crop out. Pre-Cambrian formations and those of Paleozoic age constitute the thickest sections. Recent basalt flows cap the plateau portion, except in the central part of the region. Large areas in valleys below the rim are occupied by lake-bed deposits. The valleys are aligned northwest, suggesting the possibility that a structural trough extends almost the full length of the rim southwest of the scarp. In some areas, erosion has caused recession of the escarpment for distances of a few miles to 10 or 15 miles from the major rim faults. The origin of late deposits of sodium Sulfate in the Verde basin has not been adequately, explained. As the salts are concentrated near mineralized districts on the southwest side of the basin, a possible genetic relationship between the two should be considered. Pre-Cambrian granite and basalt of probable Tertiary and Quaternary age are the igneous rocks most widely exposed in the region. Diabase dikes and sills are prominent in some areas; they were intruded probably during Late Cambrian time. A thickness of 2,000 feet of volcanic rocks of probable Cretaceous and Tertiary age is exposed in one area along the rim, but these rocks as yet have not been studied in detail. A hypothetical relationship is advanced to explain the coincidence in estimated volumes of rock erupted in the San Franciscan volcanic field and the volumes displaced by subsidence of the Verde basin. Fold structures are relatively uncommon in the region and are of small extent except the Holbrook dome northwest of Snowflake. High-angle faults, for the most part normal, are the most prominent structures identified. Faults parallel to the rim have been mapped in several areas. The inferred relations are shown on three diagrammatic sections. These faults are thought to account for the presence of two rims in the eastern division, and perhaps as many as three near Payson. Major orogeny in the region is believed to have occurred four times, as follows: (1) In the pre-Cambrian; (2) in Miocene(?) time southwest of the Mogollon escarpment; (3) in Pliocene (?) time at least in the Flagstaff area, and; (4) at or near the beginning of Quaternary time. The Laramide structures, prominent elsewhere on the plateau, are reflected only weakly in the rim region, so far as is known. Studies of perennial base flow of major streams draining southward from the rim indicate a sustained yield of about 175 cfs (cubic feet per second) measured at existing gaging stations. Runoff records and partial seepage runs show a loss of water between the upper reaches of the streams and the storage reservoirs. There is a general tendency for the water to become progressively more highly mineralized with increasing distance from headwater springs. Natural lakes, ponds, swamps, and cienagas are common in the eastern and western divisions of the rim. They lose considerable water, and some are fully desiccated each summer. They are of little use in their present condition, but might be developed as natural water catches from which recharge co