Has Geologic Rank

Connects a geologic time unit to the geochronologic rank it holds, eon, era, period or epoch. The inverse reading, Geologic Rank Of, names the same link from the rank toward each time unit of that rank. This relation gives the rank already recorded as an enum fact on each time unit a durable, comparable entity identity; it does not replace that enum fact, which keeps classifying every time unit exactly as before.