Accurate radiometric dating generally requires that the parent has a long enough half-life that it will be present in significant amounts at the time of measurement (except as described below under "Dating with short-lived extinct radionuclides"), the half-life of the parent is accurately known, and enough of the daughter product is produced to be accurately measured and distinguished from the initial amount of the daughter present in the material.The procedures used to isolate and analyze the parent and daughter nuclides must be precise and accurate.He noted that different formations contained different fossils and he could map one formation from another by the differences in the fossils.As he mapped across southern England, he drew up a stratigraphic succession of rocks although they appeared in different places at different levels.Once formations and stratigraphic sequences were mapped around the world, sequences could be matched from the faunal successions.
However, a more powerful tool was the fossilised remains of ancient animals and plants within the rock strata.
By matching similar fossils in different regions throughout the world, correlations were built up over many years.
Only when radioactive isotopes were developed in the early 1900s did stratigraphic correlations become less important as igneous and metamorphic rocks could be dated for the first time.
However, the end of the Devonian was marked by the predominance of a different life form, plants, which in turn denotes the beginning of the Carboniferous Period.
The different periods can be further subdivided (e.g.