Peixcipal Divisions of Geologic Time.« 



[A glossary of geologic terms is given on pp. 232-230.] 



Era. 



Period. 



Epoch. 



Characteristic life. 



Duration, accord- 

 ing to various 

 estimates. 



Cenozoic (re- 

 cent life). 



1 



Quaternary. 



Recent. 



Pleistocene 

 (Great 

 lee Age). 



"Age of man." Animals and plants of 

 modem types. 



MiUions of years, 

 1 to 5. 



Tertiary. 



Pliocene. 

 Miocene. 

 Oligocene. 

 Eocene. 



*'Age of mammals." Possible first appear- 

 ance of man. Rise and development of 

 highest orders of plants. 





Cretaceous. 



m 



"Age of reptiles." Rise and culmination 

 of huge land reptiles (dinosaurs), of shell- 

 fish with complexly partitioned coiled , 

 shells (ammonites), and of §rcat flying 

 reptiles. First appearance (m 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). 





Mesozoic (in- 

 termediate 

 life). 



1 



1 



Jurassic. 



i 



1 



4 to 10. 





Triassic, 



m 







■ Carbonifer- 

 ous. 



Permian. 



"Age of amphibians." Dominance of club 

 mosses (lycopods) and plants of horsetail 

 and fern types. Primitive flowering 

 plants and earliest cone-bearing trees. 

 Beginnings of backboned land animals 

 (land vertebrates). Insects. Animals 

 with nautilus-like coiled shells (ammon- 

 ites) and sharks abimdant. 







Pemsylva- 

 man. 



- 





Missi s s i p- 

 plan. 



* 





Devonian. 



C-) 



"Age of fishes." Shellfish (mollusks) also 

 abundant. Rise of amphibians and land 

 plants. 



1 



Paleozoic 

 (old life). 



Silurian. 



(&) 



Shell-forming sea animals dominant, espe- 

 ciallv those related to the nautilus (ceph- 

 alopbds). Rise and culmination of the 

 marine animals sometimes known as sea 

 lilies (erinoids) and of giant scorpion- 

 like crustaceans (eurypterids). Rise of 

 fishes and of reef-buil'dmg corals. 



17 to 25. 



1 





OrdoTician, 



■ 



Shell-form int; sea animals^ especially ceph- 

 alopods and moUusk-like brachiopods, 

 abundant. Culmination of the buglike 

 marine crustaceans known as trilobites. 

 First tr*x;e of insect life. 



1 









Cambrian. 



(&) 



Trilobites and brachiopods most charac- 

 teristic animals. Seaweeds (alp^) abun- 

 dant. No trace of land animals found. 



1 



* 



P r o t erozoic 



^ * "1*1 



Algonkian. 



(&) 



First life that has left distinct record. 

 Crustaceans, brachiopods, and seaweeds. 



1 



1 



1 



(pnmordial 

 life). 



Arcliean. 



CrystaTline 

 rocks. 



i 



Xo fossils found. 



50-f. 



The geologic record consists mainly of sedimentary bed:,— beds doposlted in water. Over large areas 

 g periods of uplift and erosion inten^ened between periods of deposition. Every such interruption m 

 x>sition in any area prodnces there what geologists term an unconformity. Many of the time divisions 

 »wn above are separatod by such unconformities—that is, the dividing lines in the table represent local 

 widespread uplifts or depressions of the earth's surface. 



Edo(5i names omitted: in less common use than those given. 



2 



