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    A digital geochronological index to act as a domain constraint for geochronology columns in corporate data tables. It decodes geochronology codes, models pseudo parent-child relationships between intervals (for example, find the epochs within a given period), allows range searches (for example, find the epochs between lower limiting and upper limiting epochs) and provides radiometric ages with ranges of uncertainty. Follows the recommendations of the BGS Stratigraphy Committee. Phanerozoic after Gradstein and Ogg (1996), with the Ordovician scheme of Fortey et al. (1995); Proterozoic after Cowie and Bassett (1989); Archaean after an unpublished decision of the Precambrian Subcommission of the IUGS. Covers the whole of geological time. Geochronological resolution usually to age level. Lower resolution in the Archaean, Proterozoic, Cambrian. Greater resolution in the Caradoc Epoch.