We simulate the effects of an intrinsically dense boundary layer at the base of the mantle on the thermal structure of D″ and the dynamics of the lower mantle. Using a finite element model of thermo-chemical convection in 2-D with a Rayleigh number of 107, we investigate the dynamic consequences of varying the density and of increasing thermal diffusivity in the basal layer to simulate enrichment in metals. Convection may occur within a stable layer at the base of the model mantle, leading to short (400 km) wavelength temperature variations that contrast with the longer wavelength (1500--2000 km) variations above the dense layer. Strong small-scale convection within the layer is not observed if the layer is only marginally stable and easily deformed. ¿ 1998 American Geophysical Union