7 PRACTICAL LESSONS IN SCIENCE, 



satisfactory, but they give us some idea of how difficult it is to 

 say how old the earth is. Geological time is divided into eras, 

 ages, and periods, and the rocks into systems and groups. And 

 once there were primary, secondary, tertiary, and quaternary 

 systems or ages. The primary and secondary have been subdi- 

 vided, and the names dropped, while the terms tertiary and quat- 

 ernary have been retained. 



Geological studies began in Europe, and the names were de- 

 rived from localities, as Devonian, from the town of Devon, and 

 Silurian from the town of Silures, in England, Jurassic from the 

 Jura mountains, etc. The first important geological work done 

 in this country was in New York. Some European names were 

 retained, but many local names were adopted, as Niagara, Ham- 

 ilton, Chemung, and other states have followed the same custom, 

 so that we have Knoxville, St. Louis, Keokuk, limestones, etc. 

 The period or group names for the Mesozoic and early Cenozoic 

 eras in this country are derived mainly from localities in the 

 western mountain region, where those rocks are most abundant. 

 With increasing investigation and knowledge the nomenclature 

 and classification will change, but the diagram on two preceding 

 pages represents in a general way the present condition of the 

 subject. 



