Significant changes in the reversal frequency of the geomagnetic field seem to occur typically at time intervals of the order of 30--50 m.y. It appears unlikely that these long-term variations in the behavior of the magnetic field would arise through fluctuations in a geomagnetic dynamo operating under uniform conditions. Instead, they may reflect changes in the conditions in the core caused by fluctuations in the temperature at the core-mantle boundary. If the geomagnetic dynamo is maintained by thermal convection in the outer core convection in the mantle may be required to transport the heat that crosses the core-mantle boundary through the mantle. Numerical investigations of thermal convection at high Rayleigh numbers admit the possibility that this convection will be intermittent, consisting of rising plumes of material, which are occasionally emitted from a thermal boundary layer at the base of the mantle. During the convective part of the intermittent cycle, temperatures at the core-mantle boundary decrease, leading to an increase in the Rayleigh number of the core. When the lower mantle again becomes quiescent, temperatures at the core-mantle boundary rise and the Rayleigh number of the core decreases. These fluctuations in the Rayleigh number of the core are likely to disturb the flow field there and may affect the frequency of geomagnetic reversals through the mechanism suggested by Levy. The seismic properties of D\ can be explained by a thermal boundary layer at the base of the mantle and periods of intermittent convection in the mantle calculated from Foster's analysis are reasonably consistent with the long-term trends in reversal frequency inferred from the geomagnetic record.