very smooth plains material (unit pvs). Analogy with the Moon suggests that most of the recorded events in the history of Mercury occurred during the first 1.5 b.y. of the planet’s life; the oldest major rock units in this quadrangle are probably at least 2 to 3 b.y. old. The geologic history of Mercury has been summarized by Guest and O’Donnell (1977), Davies and others, and Strom. Sources Prepared for the National Aeronautics and Space Administration by U.S. Department of the Interior, U.S. Geological Survey. Published in hardcopy as USGS Miscellaneous Investigations Series Map I–1408, as part of the Atlas of Mercury, 1:5,000,000 Geologic Series. Hardcopy is available for sale from U.S. Geological Survey, Information Services, Box 25286, Federal Center, Denver, CO 80225 References Gault, D. E., Guest, J. E., and Schultz, P. H., 1976, Caloris changes in Mercury’s crater populations: U.S. National Aeronautics and Space Administration, TMX-3364, p. 183–185. Gilbert, G. K., 1893, The Moon’s face, a study of the origin of its features: Philosophic Society of Washington [D.C.] Bulletin, v. 12, p. 241–292. Guest. J. E., and Gault, D. E., 1976, Crater populations in the early history of Mercury, Geophysical Research Letters, v. 3 p. 121–l 23. Guest, J. E., and O’Donnell, W. P., 1977, Surface history of Mercury: A review: Vistas in Astronomy, v. 20, p. 273–300. Hapke, Bruce, Danielson, G. E., Jr., Klaasen, Kenneth, and Wilson, Lionel, 1975, Photommetric observations of Mercury from Mariner 10, 1975: Journal of Geophysical Research, v 80, no. 17, p. 2431–2443. James, O. B., 1977, Lunar highlands breccias generated by major impacts: Soviet- American conference on Cosmochemistry of the Moon and Planets: U.S. National Aeronautics and Space Administration, Special Publication SP-370, p. 637–658. Mallin, M. C., 1976, Observations of intercrater plains on Mercury: Geophysical Rescarch Letters, v. 3, p. 581–584. Mason. R., Guest, J. E., and Cooke, G. N., 1976, An Imbrium pattern of graben on the Moon: Geologists’ Association, Proceedings, London, v. 87, part 2, p. 161–168. McCauley, J. F., 1967, The nature of the lunar surface as determined by systematic geologic mapping, in Runcorn, S. K., ed., Mantles of the Earth and terrestrial planets; London, Interscience Publications, p. 431–460. McCauley, J. F., 1977, Orientale and Caloris: Physics of the Earth and Planetary Interiors, v. 15, nos. 2–3, p. 220–250. McCauley, J. F., Guest, J. E., Schaber, G. G., Trask. N. J., and Greeley, Ronald, 1981, Stratigraphy of the Caloris