Geochemical considerations on the overall abundances for major chemical elements of the earth and CI chondrites indicate that the lower mantle is probably more enriched in silica than the upper mantle. It is concluded that about 95% of the lower mantle is composed of material having the perovskite modification and that the ''650-km'' seismic discontinuity represents a chemical boundary at which the lower mantle is about 30 to 40% richer in silicon than the upper mantle. These results support those reached earlier on the basis of geophysical constraints. Thus, about half of the earth, either by weight or volume, is composed of ferromagnesium silicate with the perovskite structure.