How are the divisions on the geologic time scale named?

Most charts refer to four major eras, basing the names on fossils associated with the rock strata. The name Precambrian means “before Cambrian”; the Paleozoic refers to “ancient (or old) life,” the Mesozoic to “middle life,” and the Cenozoic to “modern (or recent) life.” The names of most eons and eras end in “zoic” because the time intervals are usually based on animal life. For example, “paleo” means ancient and “zoic” means life-thus, paleozoic means “ancient life.”

Smaller divisions on the geologic time scale are named after places in which the rocks were found or after ancient peoples endemic to the area. For example, the Cambrian
Period is named after Cambria, the Roman name for Wales, because the sandstones and shales from this period were first described in North Wales; the Ordovician Period is from Ordovices, a Celtic people who lived in northwestern Wales where the rocks were first studied; the Devonian Period was first studied in Devonshire, England.