In Part I of this series,1 the validity of the application of the helium method to the determination of geologic time expressed in years was discussed from the analyses of a suite of Keweenawan basalts. The year was defined as the present period of the earth’s revolution around the sun, although, of course, variation of its orbital velocity may have been appreciable over geologic intervals.2 The geologic age was defined as the number of years since the beginning of accumulation of the stable end-products of the radioactive series. The parent elements of these series are present to the extent of at least a few atoms per billion in all rocks, and a determination of the amount of either of the stable end-products, lead or helium, and of the parent elements present, provides all the data necessary for computing the length of time of accumulation. It was shown, however, that . . .

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