After heating rocks in the magnetic field of the Earth to about 600°C the relative remanent magnetism Q is about 5 to 7. Q is J/KH or the quotient of remanent volume‐magnetization J at 20° in field zero by the susceptibility K times magnetic field H which produced the remanence. The remanence is impressed on the rocks during the cooling from 585°C (magnetic critical temperature of magnetite) to about 500°C. By inverting the field in this temperature‐interval the magnetization is more or less reversed. At about 480° to 500° the same field, but of opposite sign to that field which has primarily magnetized the rock, gives to the rocks a magnetization equal to zero, when measured at 20° in field zero. Not all zero‐magnetizations are equal.