40 



INTRODUCTION. 



Ideal Section of the Crust of the Earth. 



Fig. 14. 



Y Quaternary and Recent. 

 Pliocene. 



Devonian, or Old Red Sandstone. 



Ordovician and Silurian. 



Cambrian. 



Huronian. 



Laurentian. 



prising several smaller divisions or " systems." The oldest group 

 comprises the Cambrian, Ordovician, Silurian, Devonian, Carbonif- 

 erous, and Permian " systems," which are spoken of as the Primary 



