THERE is considerable physical evidence that the earth possesses a metallic core1. Jeffreys2 has estimated the viscosity of the core and finds it as low as 1010 C.G.S. units. If this value is correct, an intense thermal convection must be maintained in the core by the heat development of radioactive impurities, even if the amount of the latter is exceedingly small compared to their concentration in the earth's crust.