The northeast corner of the Mariposa 2° topographic sheet occurs within the State of Nevada (fig. 1) and includes parts of Mineral and Esmeralda Counties. In this area field crews of the U.S. Geological Survey have established 95 gravity observations in conjunction with geologic studies being made as part of the Nevada tectonic project. The principal gravity reference base station was Tonopah J, located at the Nye County Airport, 8 miles (12.9 km) east of Tonopah, Nev. The observed gravity value for this base is 979,476.15 mGals (Department of the Air Force, written commun., 1969).