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Why should the percentage of lead to uranium in zircon crystals (the key to ordinary uranium-lead, radiometric dating) depend on which geologic period they are found in?
If most of the geologic column were created during Noah's flood, would it really matter whether a zircon crystal was found in Cambrian strata or Cretaceous strata, in Jurassic strata or Tertiary strata?
Noah's flood might just as easily deposit the same crystal in one place as another. Pressure has nothing to do with it, and zircon crystals all have about the same density as their total lead content is small.
Just what is it that a Cambrian stratum has which a Cretaceous stratum lacks?
Evolution, working in tandem with geologic ages, can explain why we have index fossils, but evolution is needed to make the index fossils useful for dating strata.
While we're on this subject, you might wish to know the odds of arranging the Precambrian era, the seven geologic periods of the Paleozoic (Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, Permian), the three periods of the Mesozoic (Triassic, Jurassic, Cretaceous), and the two periods of the Cenozoic (Paleogene, Neogene or Tertiary, Quaternary) in their proper order by pure chance.
(Far from being a rubber stamp, radiometric dating would go on to revolutionize our understanding of the Precambrian.) Thus, it became possible to date strata directly from index fossils.
Note that evolution has nothing to do with how the index fossils are used to date strata!
Radiometric dating would later confirm the relative ages of the strata and tie them to absolute dates.Then the paleontologists use the geologists' dates as evidence for the age of the fossils! The relative order of the strata was first determined by the principles of stratification. Beagle, a very strong Bible believer, made it a point to have a copy of Lyell's book for the ship's library.(The principle of superposition was recognized as early as 1669 by Steno.) Reverend Benjamin Richardson and Reverend Joseph Townsend were a couple of early geologists involved in this work. Obviously, even Lyell was not pushing evolution at the time.Once the worth of index fossils had been established on the basis of stratification studies, they could logically be used to extend the correlation of rock formations to other continents.At this point in time they were simply a useful tool for correlating rock formations.