At lower mantle conditions, the thermal conductivity of non-metallic crystals is an order of magnitude lower than that of dense metallic minerals such as FeO, FeSi and Fe, which may be present in a chemical boundary layer at the base of the mantle. Because the core-mantle boundary is nearly isothermal variations in the thickness of a metal-bearing layer induce lateral temperature variations of several hundred kelvin, which in turn affect the pattern of mantle convection. Upwellings should occur where the layer is thickest; the resulting stability of the metal+silicate layer with respect to the flow may also stabilize the pattern of mantle convection, and reduce its time-dependence. ¿ American Geophysical Union 1996