The basis for assuming that like fossils indicate contemporary formation is faunal succession.
However, as previously noted, times of volcanism and metamorphism, which are both critical parts of global processes, cannot be correlated by fossil content.
In the Geologic Time Scale, time is generally divided on the basis of the earth's biotic composition, with the Phanerozoic Eon (i.e.
the Paleozoic, Mesozoic and Cenozoic Eras) representing the period of Earth's history with advanced life forms, and the Pre Cambrian (or Proterozoic and Hadean Eras) representing the period before advanced life.
Several of these subjects are examined in essays within this book. The more well-known of these is the geologic scale, which divides time into named groupings according to six basic units: eon, era, period, epoch, age, and chron.
In addition, the chronostratigraphic scale identifies successive layers of rock with specific units of time.
Presumably if all the world’s outcrops were integrated, sediments representing all of geologic time would be available for examination.
This optimistic hope, however, must be tempered by the realization that much of the record—older than 541 million years—is missing.
Its principal subdisciplines include stratigraphy, the study of rock layers, or strata, beneath Earth's surface; geochronology, the study of Earth's age and the dating of specific formations in terms of geologic time; sedimentology, the study and interpretation of sediments, including sedimentary processes and formations; paleontology, the study of fossilized plants and animals; and paleoecology, the study of the relationship between prehistoric plants and animals and their environments.
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.
Using these methods, geologists have created a geologic time scale for organizing past times in earth’s history.
Highland County igneous rock intrudes sedimentary rock (Photograph by Stan Johnson) This light-colored Highland County igneous intrusion cuts through the darker sedimentary rock.