Velocity data are compiled from measurements of nearly 1,000,000 feet of section in 500 well surveys in the United States and Canada. Average velocities for shale and sandstone sections are arranged by depth and geologic age. Deviations from the mean values are attributed to lithologic variations. The variations of velocity with depth and time are studied independently in order to develop a quantitative relationship. It is concluded that the velocity for an average shale and sand section is given by the equation V = 125.3 (ZT) 1 6/, where V is velocity in feet per second, Z is depth in feet, and T is age in years. Velocities in limestone show less definite evidence of increase with age and depth.