The addition of crustal material to the base of the continental crust, crustal underplating, may play an important role in the evolution of the continental crust and lithosphere. We have evaluated the potential for this mechanism to increase the total thickness of the crust, and using petrologic and experimental data determined the likely seismic velocity structure which results. Modeling results indicate that mantle-derived crustal material can add more than 10 km to the thickness of the crust. Depending on the composition of this underplated material and the local thermal structure, seismic velocities in this added layer will be in a range of values intermediate between velocities appropriate for crust and mantle materials (7.0--7.8 km/s), or values interpreted to be more typical of mantle materials (7.8--8.1km/s). Comparison of the predicted seismic signature with velocity data from refraction experiments and laboratory determinations of inferred lower crustal samples indicates a strong correlation between our predictions and observation. If a significant volume of the continental crust has been added by this mechanism, the age and isotopic composition of near-surface crustal rocks may be a relatively poor indicator of lower crustal and uppermost mantle evolution.