For example, isotopes with very long half lives are no good for dating rocks younger than about 100 million years.
This is because, in just 100,000,000 years of time, not enough parent will have decayed for daughter concentrations to be reliably measured.
Strata Thickness- In the late 1800s, a British geologist estimated that 75 million years has lapsed since the beginning of the Cambrian.
This estimate was based upon the maximum known thickness of strata (from Cambrian to present) divided by the average rate of sedimentation in modern environments. Joly used the salinity of ocean water to determine the age of the earth.
material and is consistent with the radiometric ages of the oldest-known terrestrial and lunar samples.