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In the Grand Canyon area the Cambrian lies beneath a huge column of strata; in California's Mojave Desert portions of the Cambrian are exposed at the surface.

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That is, an index fossil corresponded to a very specific point in the geologic column.

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.

If rock type mattered then we would expect a zircon crystal's lead content to vary dramatically within the Cambrian or Cretaceous strata according to their local rock types. Cosmic rays, on the other hand, don't penetrate that far into the earth to begin with, so we can rule them out.

The depth of burial, itself, has little to do with our mystery.

It was found that certain fossils, now referred to as index fossils, were restricted to a narrow zone of strata.

Studies done on the European continent soon demonstrated the universal validity of index fossils.

At this point in time they were simply a useful tool for correlating rock formations.

One can hardly accuse these pioneers of evolutionary prejudice.

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.

Your chances are 6.2 billion to one of getting the right order for all thirteen.

In some parts of the world the Cretaceous is found deeper than is the Cambrian in other parts of the world.

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