Traditionally, the evolutionary timescale (4.5 billion year old earth) is assumed to be correct. It serves as the standard for judging the validity of all radiometric dating results. "Geologically acceptable" dates tend to be believed; "Geologically unacceptable" dates tend to be rejected.

For potassium-argon dating, G. B. Dalrymple and M. A. Lanphere (*Potassium-Argon Dating. Principles, Techniques, and Applications to Geochronology*. San Francisco: W. H. Freeman, 1969) gave four tests of the reliability of potassium-argon dating (pp. 196-7). They are: 1. Direct comparison with other radiometric ages, 2. Direct comparison with fossils, 3. Stratigraphic sequence, and 4. Inference. Note that all but the first test reduce to whether the date fits with the evolutionary time scale, and if the other radiometric methods are chosen on the basis of their "reliability" (how well those methods fit the evolutionary time scale), the first test also reduces to a fit with that scale.

For Uranium-lead dating, Mebus A. Geyh and Helmut Schleicher (*Absolute Dating Methods*. Berlin: Springer-Verlag, 1990) accept lower discordia ages when they match (approximately) some supposed evolutionary date. However, "In many Archaean areas the lower intercept gives an age value that cannot be assigned to any known geological event. This secondary value is then viewed as meaningless." (p. 121)

For uranium disequilibrium dating, Geyh and Schleicher note eight criteria, citing D. L. Thurber, W. S. Broecker, R. L. Blanchard, and H. A. Potratz (Uranium-series ages of Pacific atoll coral. *Science* 1965:149:55-8). Their criterion # 7 is "The radiometric age should be consistent with the stratigraphic data." Geyh and Schleicher go on to state, "If even one of these criteria is not fulfilled, the results cannot be expected to be reliable." (p. 213)

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Ó 2010 Arthur V. Chadwick, Ph.D.