Austin
has already documented that, when the
mineral isochron method is applied as a test of the assumptions of radioisotopic dating, discordances inevitably result. According to Austin, four categories of discordance are found in suites of rocks with a common origin — (1) two or more discordant whole rock isochron ages, (2) a whole-rock isochron age older than the associated mineral isochron ages, (3) two or more discordant mineral isochrons from the same rock, and (4) a whole-rock isochron age younger than the associated mineral isochron ages. Our radioisotope data from the Bass Rapids diabase sill exhibit all four categories of isochron discordance. Thus the assumptions of radioisotopic dating must be questioned.