relating to or denoting the aeon that constitutes the earlier (or middle) part of the Precambrian, in which there was no life on the earth. It precedes the Proterozoic aeon and (in some schemes) is preceded by the Priscoan aeon

the main basement rocks are Archaean gneiss

the aeon that constitutes the earlier (or middle) part of the Precambrian aeon, or the system of rocks deposited during it

these organisms evolved sometime near the onset of the Archaean