Introduction: It has been estimated that our planet's age is around 4.5 billion years. In terms of a 24 volume encyclopedia, geologists divide that length first into Eras. Eras are further divided into Periods and Epochs. We can look at it this way:

Planetary Mile Stones
Table 1

Era	Period	Epoch	Volume	Headlines
Cenozoic	Quaternary	Pleistocene	1 page of book	Man appears
	Tertiary		2/3 book - 65 milyrs B.P.	Planetary cooling: Demise of Dinosaurs
Mesozoic			1 1/3 books - 225 milyrs B.P	Present shape of continents form: Age of Dinosaurs.
Paleozoic			2 books - 570 milyrs B.P	Crustal rafts on tectonic plates.
Precambrian			20 books - 80% of story	Single-cell carbon-based life, and salty seas.

For more about Periods, Epochs, and Ages click here.