used of the earliest known rocks
Refers to an earlier part of the Precambrian aeon (before or about 2,500 million years ago).
The middle era of Precambrian time, spanning the period between 3.8 and 2.5 billion years ago. Life arose on Earth during the early Archaean, as indicated by the appearance of fossil bacteria in rocks thought to be about 3.5 billion years old. Its name means "ancient."
term used to describe some of the oldest rocks of the Precambrian era, formed from 4.0 to 2.5 billion years ago.