\Ar*ch[ae]"an\, a. [Gr. 'archai^os ancient, fr. 'archh`
beginning.]
Ancient; pertaining to the earliest period in geological
history.

\Ar*ch[ae]"an\, n. (Geol.)
The earliest period in geological period, extending up to the
Lower Silurian. It includes an Azoic age, previous to the
appearance of life, and an Eozoic age, including the earliest
forms of life.
Note: This is equivalent to the formerly accepted term Azoic,
      and to the Eozoic of Dawson.