If the lower mantle is enriched in Fe¿Si relative to the upper mantle, the compositional boundary is expected to be an interface with a density jump of ~5 (¿2)% and with depth varying laterally by >100--300 km, but with changes in seismic velocities that are nearly unobservable: ~0 to -1.0 (¿2.5)% discontinuities in compressional (VP) and shear (VS) wave velocities. In contrast, the polymorphic transformation of transition-zone minerals to the perovskite-bearing assemblage of the lower mantle is estimated to produce a jump of ~5--10 percent in density, VP and VS near 650 km depth, and the depth of the seismological discontinuity due to the phase transition is relatively independent (<50--150 km variation) of temperature or horizontal position. If present, a compositional boundary is not in general expected to occur at the identical depth (~650 km) as the phase transition to the perovskite-bearing assemblage, although the depths of the two may be linked. ¿American Geophysical Union 1991