constant, seems very unusual. This suggests that what is occuring is some kind of a mixing phenomenon, and not an isochron reflecting a true age. "Processes of rock alteration may render a volcanic rock useless for potassium-argon dating . . We have analyzed several devitrified glasses of known age, and all have yielded ages that are too young. Some gave virtually zero ages, although the geologic evidence suggested that devitrification took place shortly after the formation of a deposit." *J.F. Evernden, et. al., "K / A Dates and Cenozoic Mannalian Chronology of North America," in American Journal of Science, February 1964, p. 154. Why a low anomaly percentage is meaningless One of the main arguments in favor of radiometric dating is that so many dates agree with each other, that is, with the date expected for their geologic period. But it's not evident how much support this gives to radiometric dating. If a rock dates too old, one can say that the clock did not get reset. If it dates too young, one can invoke a later heating event. Neither date would necessarily be seen as anomalous. If lava intrudes upon geologic period X, then any date for the lava of X or later will not be seen as anomalous. And even if the date is one or two geologic periods earlier, it may well be close enough to be accepted as non-spurious. If one does not know the geologic period of a rock by other means, then of course