Radiometric dating of ordovician period
Modern methods of dating the earth and the major events in the evolution of life depend on understanding radioactive decay processes.
Certain radioactive isotopes in rocks are the driving mechanisms for reliable "atomic clocks." For instance, uranium-238 decays through several steps into lead-206.
While he actually did not have the full necessary data available, Halley used this approach to conclude that the earth is minimally 100,000,000 years old.
proposed that different layers of rock came from specific periods of geologic time.
Further, he observed that the layers that were deeper down were older.