Ages of five whole rock samples from the North Mountain Basalt, Nova Scotia, determined by Isotopes, Inc. in 1967, were discordant and indicated the effect of initial argon with a high 40Ar/36Ar ratio. Reanalysis of the same samples has yielded a 191 Ma isochron with a low initial 40Ar/36Ar ratio. Although the isochron age is similar to the value previously assigned to the basalt, the result suggests that application of the K–Ar isochron method may improve the precision of K–Ar age determination considerably.

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