Reply to a discussion of a paper by R.D. Russell (Geoscience Abstracts 1-1950). The writer's use of the term fractionation has been in its broadest sense so as to include all processes leading to the separation of Pb isotopes. There is reason to suspect that physico-chemical fractionation of Pb isotopes takes place in nature. Whether age values refer to the time Pb was released from its source rock or to the time it was deposited as a Pb mineral is of minor significance.

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