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Furthermore, useful fossils are either rare or totally absent in rocks from Precambrian time, which constitutes more than 87 percent of Earth history.Precambrian rocks must therefore be correlated by means of precise isotopic dating.Using fossils simply for identification purposes, Smith constructed a map of the various surface rocks outcropping throughout England, Wales, and southern Scotland.Smith’s geologic map was extremely crude, but in its effect on Earth study it was a milestone.Nevertheless, there is no greater testimony to the validity of fossil-based stratigraphic geology than the absolute dates made possible through radioactive measurements.Almost without exception, the relative order of strata defined by fossils has been confirmed by radiometric ages.
However, as previously noted, times of volcanism and metamorphism, which are both critical parts of global processes, cannot be correlated by fossil content.
Suppose there exist a number of fossil-bearing outcrops each composed of sedimentary layers that can be arranged in relative order, primarily based on superposition.
Suppose, too, that all the layers contain a good representation of the animal life existing at the time of deposition.
Unlike the principles of superposition and crosscutting, faunal succession is a secondary principle.
That is to say, it depends on other sequence-determining principles for establishing its validity.
Correlating two separated outcrops means establishing that they share certain characteristics indicative of contemporary formation.