Discusses the use of the rate of radioactive decay of minerals as a criterion for determining the geologic age of crystalline basement rocks, noting potential sources of error, due largely to lack of consideration of petrogenetic factors, and the reliability of data based on analyses of monazite, whose field of stability is greater than that of other uranium minerals. The Precambrian-early Paleozoic chronology of Brazil is summarized on the basis of age determinations of radioactive minerals. Analyses of fresh and weathered djalmaite from Minas Gerais and monazite from numerous localities are included.

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