A low compressional-wave velocity region in the midcrust below the San Francisco Mountain stratovolcano, Arizona, has been detected by the teleseismic P residual technique. This region is approximately 6 km wide, lies between elevations of 9 km and 34 km below sea level, and has a compressional velocity reduction of more than 6% with respect to the surrounding rocks. Several mechanisms are found to be quantitatively sufficient to produce such a feature. These include (a) a cool silicic pluton enclosed in a more mafic crust, (2) high temperature (near but below the solidus) in a quartz-bearing rock in the low-velocity region, (3) high density of water-filled cracks having pore pressures nearly equal to lithostatic pressure, and (4) the presence of melt, either in intergranular pores or in crystal-poor dikes.