There are two main methods determining a fossils age, relative dating and absolute dating.
Relative dating is used to determine a fossils approximate age by comparing it to similar rocks and fossils of known ages.
Stratigraphy is the oldest of the relative dating methods that archaeologists use to date things.
Stratigraphy is based on the law of superposition--like a layer cake, the lowest layers must have been formed first.
Within the context of the Phanerozoic Eon, geologists beginning in the late 1700's recognized that fossils appeared in an orderly fashion in stratigraphic units.
Moreover, these geologists recognized that the fossilized biota demonstrated rather large changes in overall composition and showed both similarities with, and differences from living taxonomic groups.
Scientists can use certain types of fossils referred to as index fossils to assist in relative dating via correlation.
Two broad categories of dating or chronometric techniques that archaeologists use are called relative and absolute dating. Relativism in philosophy first recorded 1865 (relativist is from 1863). of referre "to refer." Meaning "person in the same family" first recorded 1657; the adj. Relatively "in relation to something else" is recorded from 1561.The Modern Geologic Time Scale, as shown above, documents intervals of geologic time relative to one another, and has been continuously developed and updated over the last two centuries.In addition to the relative dating of periods in Earth's history for which we have rocks preserved, geologists are now able to assign absolute age dates to critical intervals. The hall-mark of all Christians is a relative purity, not of actions, but of soul.