First some big divisions:
Phanerozoic -- 543 my to present Precambrian -- 3800 my to 543 my Hadean -- pre 3800 myYou almost never hear the term Hadean used. The current estimate for the age of the earth is 4600 my. The Precambrian is often used and is sometimes broken down as follows:
Proterozoic 2500 to 543 my Archaen -- 3800 to 2500 myThese can be broken up as follows, but rarely are:
Late Proterozoic -- 900 to 543 my Middle Proterozoic -- 1600 to 900 my Early Proterozoic -- 2500 to 1600 my Late Archean -- 3000 to 2500 my Middle Archean -- 3400 to 3000 my Early Archean -- 4600 to 3400 myYou don't often hear the word "Phanerozoic" used as geologists tend to be much more specific. There are 3 big divisions of the Phanerozoic:
Cenozoic -- 65 my to present Mesozoic -- 248 to 65 my Paleozoic -- 543 my to 248 myThe Paleozoic gets broken up as follows:
Permian -- 290 to 248 my Pennsylvanian -- 323 to 290 my Mississippian -- 354 to 323 my Devonian -- 417 to 354 my Silurian -- 443 to 417 my Ordovician -- 490 to 443 my Cambrian -- 543 to 490 myThe Mesozoic gets broken up as follows:
Cretaceous -- 144 to 65 my Jurassic -- 206 to 144 my Triassic -- 248 to 206 myThe Cenozoic gets broken up as follows:
Holocene -- 0.01 my to present Pleistocene -- 1.8 to 0.01 my Pliocene -- 5.3 to 1.8 my Miocene -- 23.7 to 5.3 my Oligocene -- 33.7 to 23.7 my Eocene -- 54.8 to 33.7 my Paleocene -- 65 to 54.8 myThe Holocene and Pleistocene are referred to together as Quarternary.
Sometimes Miocene and Pliocene get lumped together as "Neogene", but this is rare.
Similarly, Paleocene, Eocene, and Oligocene rarely get lumped as "Paleogene".
There is always some individual eager to use some seldom used term in an effort to try to impress you.
Taken from page 14 of "Aerial Geology" by Mary Caperton Morton.
Tom's Geology Info / tom@mmto.org