32 HISTORICAL GEOLOGY • 



Table of Main Geological Divisions 

 Era and group Period and system 



f Cretaceous. 

 Mesozoic { Jurassic. 

 [ Triassic. 



Permian. 



Pennsylvanian (Upper Carboniferous). 



Mississippian (Lower Carboniferous). 

 Paleozoic { Devonian. ^ 

 | Silurian. 

 j Ordovician. — of 

 [ Cambrian. 



Proterozoic j Algonkian. 



Archeozoic { Archean. 



The names of eras follow a definite plan depending upon the 

 great life stages. Thus Archeozoic means literally " primitive or 

 beginning life"; Proterozoic means "earlier or less primitive 

 life"; Paleozoic means " ancient life"; Mesozoic means "inter- 

 mediate life;" and Cenozoic means "recent life." The period 

 names do not follow such a definite plan of nomenclature, 

 various ideas being represented. These names will be explained 

 when the different periods are taken up for discussion. 



Comparison of Human and Geologic History 



One of the most striking differences between human and geo- 

 logic history is the extreme brevity of the one as compared with 

 the vast time represented b}^ the other. Human history is to be 

 measured by some thousands of years, while geologic history must 

 be measured by at least tens of millions of years. A recent event, 

 geologically speaking, like that of the building of the Coast Range 

 Mountains, or the carving out of a tremendous canyon like the 

 Grand Canyon of the Colorado in Arizona, required some hundreds 

 of thousands, if not a few millions, of years. Human history is 

 roughly divided into certain ages according to the predominant 

 influence of some person, nation, principle, or force. Thus we 

 speak of the "Age of Pericles," the "Roman Period," the "Age 

 of the French Revolution," or the "Age of Electricity." Geologic 





