The Precambrian: the Archean and Proterozoic eons

Archean and Proterozoic time scales are currently defined chrono-metrically, with subdivisions into eras and periods being defined and allocated boundaries in terms of a round number of millions of years before present. Isotope stratigraphy is increasingly used to identify tectonic, chemical, and biological changes. The Neoproterozoic Era is characterized by at least two, and possibly four, severe and extensive glaciogenic events; for this era, chronostratigraphic subdivisions following established Phanerozoic practices are possible.