ABSTRACT: The five-point Rb-Sr whole-rock isochron age of 1.07 Ga for the diabase sill at Bass Rapids, Grand Canyon, has been regarded for 20 years as an excellent example of the application of conventional radioisotopic dating.  However, our new K-Ar, Rb-Sr, Sm-Nd and Pb-Pb radioisotope data from eleven whole-rock samples (eight diabase, three granophyre) and six mineral phases separated from one of the whole-rock diabase samples yield discordant whole-rock and mineral isochron “ages.”