The calculation of the atomic weights of rock leads from a new series of thorium-uranium ratios on basaltic (3.3) and granitic (2.8) rocks from widely separated localities has shown that the variation to be expected is 207.18 to 207.21, which is exactly what is found from recent isotopic analyses of a representative series of ore leads. The new results indicate that the values calculated by Holmes are too low, probably because of his including some unreliable data. This makes unnecessary the conclusion reached by Holmes that ore leads must come from a deep magmatic source, and this combined with the critical analysis of the problem by Graton would indicate that lead ores are probably derived from basaltic or granitic rocks or their magmas.The results of thorium and uranium analyses on a series of different rocks and comparisons, using these data, with Holmes' calculations and with the chemical and physical determinations of the atomic weights of ore leads are presented in tabular form.

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