Compressional and shear wave velocities were measured at confining pressures up to 200 MPa for 12 core samples from the depth interval of 600--2600 m in the California State 2--14 borehole. Samples were selected to represent the various lithologies, including clean, heavily cemented sandstones, altered, impermeable claystones, and several intermediate siltstones. Velocities measured at ultrasonic frequencies in the laboratory correspond closely with velocities determined from acoustic waveform logs and vertical seismic profiles. The samples exhibit P wave velocities around 3.5 km/s at depths above 1250 m but increase to nearly 5.0 km/s at 1300 m in depth. Further increases with depth result in compressional wave velocity increasing to nearly 6.0 km/s. These increases in velocities are related to systematic variations in lithology, microstructure, and hydrothermal alteration of originally clay-rich sediments. Scanning electron microscope observations of core samples confirm that local core velocities are determined by the combined effects of pore size distributions and the proportion of clays and alteration minerals such as epidote present in the form of pore fillings and veins. ¿ American Geophysical Union 1988