Synopsis of Common Rocks 



1. CENOZOIC TIME: THE AGE OF MAMMALS. 



Quaternary or Pleistocene. 



Man assumes importance, particularly in the latter part of the Quaternary 

 which is known as the Recent Period. Glacial Period in the first half. 



Tertiary: Pliocene, Miocene, Oliaocene, Eocene. 

 Mammals develop in remarkable variety, and to great size, while reptiles 

 diminish. 



2. MESOZOIC TIME: THE AGE OF REPTILES. 



Cretaceous. 



Birds begin to be important; reptiles continue; and higher mammals 

 appear; land plants and insects of high type. 



Jurassic. 

 Reptiles and amphibia predominate. 



Triassic. 



Amphibia and reptiles develop remarkably; low forms of mammals ap- 

 pear. 



3. PALEOZOIC TIME: AGE OF INVERTEBRATES. 



Permian, Carboniferous. 

 Land plants assume great importance. 



Devonian. 



Fishes are abundant. They began in the Silurian and continue, though 

 with many changes, to the present time. 



Silurian, Ordovician. 



Invertebrates prevail. They continue abundant to the present time, but 

 are of different kinds. 



Cambrian. 

 No forms higher than invertebrates. 



4. PRE-CAMBRIAN TIME. 



Few fossils known. 

 Algonkian, Archean. 

 Mostly metamorphic rocks ; perhaps, in part, original crust of earth. 



The data in this section has been derived, in large part, from the College 

 Physiography of Tarr and Martin, which is recommended as a reference 

 work for those who desire to make first-hand studies of earth features. 



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