Thorium and uranium determinations by the thoron and radon methods have been made on 19 composite samples from 1500 granitic and intermediate type rocks from various parts of North America. Average Th/U ratios of 3.4 and 4.0 were found for granitic and intermediate type rocks respectively. Such ratios lend no support to Arthur Holmes' theory that lead ores are derived from a source in the substratum below the zone of ordinary magmatic activity. Some regional variations in average Th/U ratios were noted, relatively more uranium being found in the northern part of the continent.
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Senftle et al. (1947) studied this question.
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