Of or relating to the earliest known rocks formed during the Precambrian Eon WordNet 3.0 & Open English WordNet

Of, or relating to the geologic eon from about 3,800 to 2,500 million years ago; comprises the Eoarchean, Paleoarchean, Mesoarchean and Neoarchean eras; marked by an atmosphere with little oxygen, the formation of the first continents and oceans and the emergence of simple life. Wiktionary

Of eon from 3,800 to 2,500 million years ago. Wiktionary (translation)