Eon and Eonothem
Eon is the longest primary time unit used in geology. Based on the emergence of fauna and flora, geologic history can be divided into: (1) the Archean Eon, which lacks fossils; (2) the Proterozoic Eon, which has numerous fossils; and (3) the Phanerozoic Eon, which has abundant fossils. An eon can be subdivided into eras. An eonothem is all of the rocks that formed during a certain eon. It is the largest unit of chronostratigraphic units; examples are the Archean Eonothem, Proterozoic Eonothem and Phanerozoic Eonothem.