A discussion of methods which have been used for estimating the duration of geologic eras and the age of the earth with emphasis on new techniques which have been recently developed. The new techniques make use of refined methods for the precise determination and isolation of micro quantities of trace elements and will lead eventually, it is hoped, to the formulation of a precisely dated geologic time scale. The analytical techniques known as "isotope-dilution techniques" make use of separate isotopes and mass spectrometry and make possible the precise determination of trace-element concentration at levels at 0.001 part-per-million.

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