Compressional velocity was measured to 300¿C and 8 kbar in 14 dry rocks ranging from zeolite facies sandstone to jadeitized metagraywacke from the Franciscan complex of central and northern California. At pressure and temperature with an error of less than 1%. For all the rocks taken together, velocity may be expressed as a linear function of temperature, pressure, density, and porosity or as a linear function of pressure, temperature, and bulk density; in either case, root-mean-square velocity residuals are about 1.5%. Critical thermal gradients for individual rocks average about 15¿ C/km. Small increases in bulk density due to elimination of porosity or crystallization of dense minerals can maintain constant velocities in the face of supercritical geothermal gradients. The laboratory data are consistent with available seismic refraction and rock density measurements in the Franciscan terrane of central California.