Principal Divisions of Geologic Time.« 



[A glossary of geologic terms is given on pp. 13^136.] 



Era. 



Period. 



Epoch. 



Characteristic life. 



Duration, accord- 

 ing to various 

 estimates. 



Cenozoic (re- 

 cent life). 



Quaternary. 



Recent. 1 

 i'leistocene 

 (Great 

 Ice Age). 



1 



"Age of man." Animals and plants of 

 modem types. 



Millions of years. 

 1 to5. 



Tertiary. 



Pliocene. 

 Miocene. 

 Oligocene. 

 Eocene. 



*'Age of mammals." Possible first appear- 

 ance of man. Rise and development of 

 highest orders of plants. 



- 



Cretaceous.* 



i 

 1 



(») 



"Age of reptiles." Rise and culmination 

 of huge land reptiles (dinosaurs), of shell- 

 fish with complexly partitioned coiled 

 shells (ammonites), and of great flying 

 reptiles. First appearance (in Jurassic) 

 of birds and mammals; of cycads, an 

 order of palmlike plants (in Triassic); 

 and of angiospermous plants, among 

 which are palms and hardwood trees 

 (in Cretaceous). 



1 





Mesozoic (in- 

 termediate 

 life). 



Jurassic. 



(^) 



4 to 10. 





Triassic. 



1 (^) 







i 



Carbonifer- 

 ous. 



Permian. 



1 



"Age of airiphibians." Dominance of club 

 mosses (lycopods) and plants of horsetail 

 and fern t3i)cs. Primitive flowering 

 plants and earliest cone-bearing trees. 

 Begmnincrs of backboned land animals 

 (land vertebrates). Insects. Animals 

 with nautilus-like coiled shells (ammon- 

 ites) and sharks abundant. 



n 





Fennsylva- 

 nian. 



1 

 1 





Missi s s i p- 



pian. 





1 



Devonian. 



('>) 



"Age of fishes." Shellfish (mollusks) also 



abundant. Rise of amphibians and land 

 plants. 





Paleozoic 

 (old life). 



1 



1 

 1 



Silurian, 



(^) 



Shell-forming sea animals dominant, espe- 

 cially those related to the nautilus (ceph- 

 fllopods). Rise and culmination of the 

 marine animals sometimes known as sea 

 lilies (crinoids) and of giant scorpion- 

 like crustaceans (eurvpterids). Rise of 

 fishes and of reef-building corals. 



17 to 25. 



1 



Ordovician. 



(») 



Shell-forming sea animals, especially ceph- 

 alopods and mollusk-like brachiopods, 

 abundant. Culmination of the buglike 

 marine crustaceans known as trilobites. 

 First trace of insect life. 





1 



1 

 1 



1 



Cambrian. 



(?) 



1 



Trilobites and brachiopods most charac- 

 teristic animals. Seaweeds (algae) abun- 

 dant. No trace of land animals foimd. 



1 



1 



P r 1 erozoic 

 (primord iai 



life). 



Algonkian. 



(.») 



First life that has left distinct record. 

 Crustaceans, brachiopods, and seaweeds. 





Archean. i 



Crystalline 

 rocks* 



No fossils found. 



50 -f-. 



o The geologic record consists mainly of sedimentary beds — ^bods deposited in water. Over large art^as 

 long periods of uplift and erosion intervened between periods of deposition. Every such interruption in 

 deposition in any area produces there what geologists term an unconformity. Many of the time divisions 

 shown above are separated by such unconformities — that is, the dividing Ixnes in the table represent local 

 or widespread uplifts or depressions of the earth's surface. 



ft Epoch names omitted; in less common use than those given. 



2 



