From Wikipedia on Geologic Time Scale:

Quote:

The identification of strata by the fossils they contained, pioneered by William Smith, Georges Cuvier, and Alexandre Brogniart in the early 19th century, enabled geologists to divide Earth history more finely and precisely. It also enabled them to correlate strata across national (or even continental) boundaries. If two strata (however distant in space or different in composition) contained the same fossils, chances were good that they had been laid down at the same time. Detailed studies of the strata and fossils of Europe produced, between 1820 and 1850, the sequence of geological periods still used today.



Quote:

Advances in the latter part of the 20th century allowed radioactive dating to provide relatively firm dates to geological horizons. In the intervening century and a half, geologists and paleontologists constructed time scales based solely on the relative positions of different strata and fossils.



My contention is that while radiometric dating methods are used that there are so many various ways of tesing and, in many cases, a variety of results are obtained. When strata are tested they are tested with a theory already in place. That theory is that the strata must be within a certain age. Tests that do not give a date within that age are disregarded (for a variety of reasons) and tests that get within the ballpark are often accepted. There may be valid reasons for ignoring some of the tests, but the "fixed theory" of the age of the strata was set long ago. The idea of dating the strata by the fossils they contain is "set in stone" and thus the dating methods rely on this as a means to get a general idea of the age of the strata.

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