New data on thermal expansiviity, &agr;, of iron to over 400 kbar in laser- and electrically-heated diamond anvil cells and on MgO and forsterite from spectroscopic data to over 200 kbar and previously measured adiabats [Boehler, 1982>, imply that &agr; at high pressure may be described by a constant value of (∂ln&agr;/∂lnV)T=5.5¿0.5. This behavior is supported by previous &agr; measurements at lower pressures on a large variety of materials. Densities of magnesio-w¿stite and silicate perovskite in a pyrolitic ratio calculated at lower mantle conditions using a finite strain equation and &agr; at high pressures, using the above systematics, match those in PREM through the entire lower mantle, making a strong case for a uniform mantle. ¿ American Geophysical Union 1989