Rock classification is organized according to different schemes which do not have one-to-one correspondence; that is, lithostratigraphic and geochronologic or chronostratigraphic subdivisions sometimes cross each other. Lithostratigraphic subdivisions (based on lithologic character, also called facies, q.v.) are (from largest to smallest)Group, Formation, Member, and Bed. Geochronologic subdivisions (based on age relations) are Eon, Era, Period, Epoch, Age, and Chron; these correspond to the Chronostratigraphic subdivisions Eonothem, Erathem, System, Series, Stage, and Chronozone.
There are also biostratigraphic subdivisions (based on fossil content), classification by residual magnetism (magnetostratigraphy, q.v.), chemical properties (chemostratigraphy), etc. Terminology is standardized by the International Subcommission on Stratigraphic Classification of the IUGS (Hedburg, 1976).