Forms of dating rocks
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.
Presumably if all the world’s outcrops were integrated, sediments representing all of geologic time would be available for examination.
The basic conceptual tool for correlation by fossils is the index, or guide, fossil.
Correlating two separated outcrops means establishing that they share certain characteristics indicative of contemporary formation.
The most useful indication of time equivalence is similar fossil content, provided of course that such remains are present.
The more ways in which two rocks are physically alike, the more likely it is that the two formed at the same time.
Only a partial listing of physical characteristics is necessary to indicate the breadth of approach in this area.
Nevertheless, there is no greater testimony to the validity of fossil-based stratigraphic geology than the absolute dates made possible through radioactive measurements.