The determination of the age of common rocks by the ``helium method'' necessitates accurate measurements of the radium content. The method of compensating the background by the use of two opposed ionization chambers is applied to radium determinations and the apparatus has an observational limit of 5.5×10—14×n—½g radium fornhourly readings. With the thorium content determined as in the preceding article a summary of results for a suite of rocks from a single horizon and the present status of a geological time scale is given.
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