Abstract
The steady spontaneous transmutation of a radioactive isotope into a stable daughter isotope provides a means for determining the length of time the accumulation of daughter atoms has been maintained in association with its parent source - the radioisotope age of the mineral that contains the parent and daughter atoms. It is reasonable to expect that a radioisotope age for a mineral formation should specify the length of time that formation has been in existence, and also the minimum age of any fossils that may be associated with it. On this basis a quantitative geologic time scale has been developed (Harlan et al. 1964, 1971).