Five-Billion-Year Clock, Patrick M. Hurley, _Saturday Evening Post_, 234: 26 (March 18, 1961). Geologic Time Scale, J. Laurence Kulp, _Science_, 133: 1105 (April 14, 1961). Geology, Reginald A. Daly, _Scientific American_, 183: 36 (September 1950). Tracks of Charged Particles in Solids, R. L. Fleischer, P. B. Price, and R. M. Walker, _Science_, 149: 383 (July 23, 1965). How Old Is It?, Lyman J. Briggs and Kenneth F. Weaver, _National Geographic Magazine_, 114: 234 (August 1958). Moving Picture of the Last Ice Age, Richard Foster Flint, _Natural History_, 66: 188 (April 1957). Modern Methods for Measurement of Geologic Time, E. J. Zeller, _Mineral Information Service_, 18: 9 (January 1965). Single copies are $0.10 from Division of Mines and Geology, Ferry-Building, San Francisco, California 94111. Fluted Projectile Points: Their Age and Dispersion, C. Vance Haynes, Jr., _Science_, 145: 1408 (September 25, 1964). Unraveling the Age of Earth and Man, E. L. Simons, _Natural History_, 76: 52 (February 1967). Radiocarbon Dating and Archeology in North America, F. Johnson, _Science_, 155: 165 (January 13, 1967). Fission-track Dating of Bed I, Olduvai Gorge, R. L. Fleischer and others, _Science_, 146: 72 (April 2, 1965). Lead Isotopes and the Age of the Earth, G. R. Tilton and R. H. Steiger, _Science_, 150: 1805 (December 31, 1965). Strontium-Rubidium Age of an Iron Meteorite, G. J. Wasserburg and others, _Science_, 150: 1814 (December 31, 1965). [Illustration: THE COVER] U. S. Geological Survey scientists prepare acetylene gas, made from the carbon-14 in a geological specimen, in a vacuum line. This gas will be fed into a proportional counter to determine the age of the specimen by its ¹⁴C count. (See “Carbon-14 Counting” beginning on page 12.) [Illustration: THE AUTHOR] Geophysicist Henry Faul, an authority on nuclear dating, is professor of Geophysics and chairman of the Department of Geology at the University of Pennsylvania, Philadelphia. He holds a doctoral degree from the Massachusetts Institute of Technology, and has taught at the Universities of Strasbourg and Bern in Europe. Dr. Faul was a geophysicist with the Manhattan Project during World War II, and was formerly chief of the Radiation Laboratory, U. S. Geological Survey, Denver, Colorado. For many years he was engaged in geological age determination work with the Geological Survey and the Carnegie Institution of Washington in Washington, D. C. PHOTO CREDITS Frontispiece courtesy Anthropology Department, Southern Methodist University (SMU) Cover photo courtesy U. S. Geological Survey (USGS)