Based upon the variation of isotopic composition of lead ores from the same site, the age of the earth is estimated to be 4.55 billion years ( b. y. = 10 9years). By assuming a fixed heterogeneity of two radioactive elements, thorium and uranium, we can obtain a self-consistent primeval lead with 204Pb: 206Pb: 207Pb: 208Pb = 1: 9.05: 9.81: 29.45 . These figures demonstrate for the first time that the isotopic composition of earth's primeval lead does not differ essentially from that of primeval lead of meteorites or troilite lead. It is the chief advantage of this approach that no knowledge of deposition time for the vast majority of samples is required. Although common igneous rocks are accountable for lead ores, mafic rock is of greater importance than felsic.