The Coast Ridge Belt (CRB, Santa Lucia Mts., central California) comprises mid-crustal rocks (750--800¿C and 0.8 GPa) of the California magmatic arc. We estimated the bulk composition of the CRB and converted our results to seismic velocities expected at the observed pressures and temperatures. This transformation allows us to compare calculated velocities for the CRB with in situ measurements in similar arcs. The bulk composition of this arc section changes abruptly at 25 km depth from a granodiorite to a quartz-diorite or diorite. These data are in agreement with geophysical results from other Cordilleran batholiths, and suggest 1.5 to 2 times thicker felsic columns than usually interpreted for modern continental arcs, and a relatively sharp transition between a felsic upper crustal batholith, and a mafic deep crust. This implied rheological boundary may have significant implications for intracrustal faulting or convective removal of the roots of batholiths.