CHAPTER XV 

 THE CAMBRIAN PERIOD 



The Paleozoic Era. — Lying above the Pre-Cambrian formations 

 is the Paleozoic group, which includes the following systems : 



Younger Paleozoic 



Characterized by the presence of verte- 

 brates, both fishes and amphibians 



7. Permian 



6. Pennsylvanian I Carboniferous 



5. Mississippian I 



4. Devonian 



Older Paleozoic ( 3. Silurian 



Characterized by the scarcity of verte- I 2. Ordovician 

 brates [ 1. Cambrian 1 



The first three of these systems (the Cambrian, Ordovician, and Si- 

 lurian) are sometimes grouped together as the older Paleozoic, since 

 they are characterized by invertebrate life, vertebrate remains being 

 absent in the first half and rare in the second half. The younger 

 Paleozoic (Devonian, Mississippian, Pennsylvanian, and Permian) is 

 characterized by the higher forms of life, such as fishes, amphibians, 

 and reptiles, although invertebrates were as abundant as in the older 

 Paleozoic. These seven systems 2 are not of equal length, nor are 

 they of equal importance in the evolution of life, but may be recog- 

 nized in any portion of the world in which they occur by their pecul- 

 iar fauna. Locally, the systems are often clearly marked by un- 

 conformities (p. 270), but it is upon differences in the faunas that the 

 separations are ultimately based (p. 417). 



The maximum thickness of the Paleozoic rocks in Europe is 

 estimated at 100,000 feet, and in the Appalachian region of this 

 country a maximum up to 40,000 feet is exposed. The duration of 

 the Paleozoic era was immense, exceeding that of all subsequent time. 



1 Ozarkian is a proposed system which includes the Upper Cambrian and part of the Lower 

 Ordovician. 



2 Schuchert, Chas., — The Delimitation of the Geologic Periods, illustrated by the paleogeog- 

 raphy of North America: International Geological Congress, 191 3. 



401 



