Compositions of initial, generally basaltic, rocks of geosynclines are virtually identical regardless of their time and place of formation within the past 3,500 m.y. This constancy of composition through time and place is matched by a similarity of the entire stratigraphic column in each of the studied geosynclines of different age and location. These observations lead to the conclusion that the major element content of the upper mantle has been essentially unchanged during the past 3,500 m.y. except by processes related to geosynclinal development and orogeny. The resulting compositional changes are caused largely by gradual segregation of new continental crust out of the mantle during geosynclinal, orogenic, and postorogenic stages. General enlargement of cratons may also explain the inception of major miogeosynclinal sedimentation in Proterozoic time and platform sedimentation in Paleozoic time.