The three geologic eras are the PALEOZOIC, MESOZOIC and CENOZOIC - from the Greek for ancient, middle and recent life.  They are divided into 11 periods most of them named for the places where rocks from the period were first discovered.

The Cambrian Period (570-500 million years ago) is named for Cambria, or Wales.  The next two periods also received Welsh names: Ordovician and Silurian for two Welsh tribes, the Ordovicies and the Siluries.

The Devonian is named for Devonshire, England, and the Cretaceous comes from "creta", Latin for chalk, referring to the White Cliffs of Dover, England.

The Jurassic is named for the Jura Mountains in Germany, and the Permian for Perm is Russia's Ural Mountains.

The Triassic got its name because it was easily divided into three parts.  In North America the Carboniferous  (carbon = coal) is subdivided into the Mississipian, named for the Mississippi River and the Pennsylvanian, named for the coal deposits first discovered in the state of Pennsylvania.

Reference: The Pterodactyl, The Roamin R&M Club, 04/03, via Quarry Guips, 11/03, and printed in the Rocky Mountain Federation News, 01/04. (modified).

A NEW ERA - 2006
Divisions in geologic time are based on major changes in the types of life forms that inhabited Earth.  The newest period to be added to the time line is the Ediacaran (ee-dee-AH-kah-ren) Period.  It represents the time when scientists believe the first soft-bodied animals appeared on Earth.