The bulk composition of the sedimentary rock-cover in several regions was estimated by using published chemical analyses of the component rock types. This composition changes as a function of age. Archean sedimentary assemblages are comparatively richer in Al2O3, Na2O, and MgO than most Proterozoic and Phanerozoic assemblages. The younger rock assemblages are progressively richer in SiO2, K2O, and (generally) CaO. These compositional differences define secular trends that probably reflect evolutionary changes in the composition of the Earth's upper crust, atmosphere, and hydrosphere, although the trends may partly be the result of either secondary processes (such as diagenesis, metamorphism, and sediment recycling) or selective preservation of particular rock types because of structural and stratigraphic position.