Very long span of geologic time; in formal usage, a portion of geologic time of the second-greatest magnitude (eons are longer). Three eras are recognized: Paleozoic, Mesozoic, and Cenozoic. Because of the difficulties involved in establishing accurate chronologies, the Precambrian, or earliest, eras are classified independently. An era is composed of one or more geologic periods.

This entry comes from Encyclopædia Britannica Concise.
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