New P and S wave seismic data from the Basin and Range province-Colorado Plateau transition zone allow a better determination of the composition of the lower crust than would be possible from P wave data alone. The data were collected in 1987 by recording large seismic refraction shots into stationary reflection spreads during the U.S. Geological Survey's Pacific to Arizona Crustasl Experiment. Significant seismic observations about the lower crust include (1) strong P wave reflections at its top, (2) moderate-strength S wave reflections from the entire lower crust, (3) an average P wave velocity of 6.6¿0.15 km s-1, and (4) an average Poisson's ratio of &sgr;=0.27¿0.02. Using these constraints and physical property measurements from laboratory rock samples, we conclude that the average composition of the lower crust in this region is intermediate-to-mafic and that mafic underplating has been minor. Consequently, the Cenozoic-age mafic and ultramafic xenoliths that are found in the Transition Zone must have resided in the upper mantle or in a thin transition zone at the base of the crust, since these rocks have higher P wave velocities and Poisson's ratio than-observed seismically. ¿American Geophysical Union 1990