Time can be measured by observing unidirectional and nonreversible processes, or processes that proceed with a fixed periodicity. Geologic time is measured by observations of rock features that give evidence of previous existence of unidirectional or periodically repeated processes. Periodically recurrent processes do not provide world-wide time standards. The only natural processes that are of a unidirectional and nonreversible nature and that leave universally occurring testimony in the rocks are radioactive decay and evolution of life. The study of past life forms and their distribution in the rocks (paleontology and biostratigraphy) provides a reference system for determining the order of succession of geologic events from the Cambrian to the Recent. Radioactive dating supplies an approximative reference grid of dates in terms of absolute time.
The biostratigraphic zone, which is an assemblage of rocks containing a defined fossil fauna or flora, is the basic and generally the smallest biostratigraphic unit on which world-wide correlations can be established. Application of biostratigraphy to geochronology is known as biochronology. Different plant and animal phyla change in biochronological value during their development, and at different periods during earth history, different groups of animals and plants attained varying degrees of biochronologic importance. The biochronological standard, based for each period on biostratigraphically important groups of fossils, is known as the orthochronologic standard. The basic unit of the orthochronologic standard is the time unit corresponding to the deposition of a biostratigraphic zone.
A multiple biostratigraphic terminology is found unnecessary. Zone is accepted as the basic biostratigraphic unit, moment is recommended for the time unit during which a zone was deposited. Instant is suggested for a true “time plane” in stratigraphy. Biochron is suggested for the duration or survival time of taxonomic units, down to the species.