GEOLOGIC HISTORY. 447 



also possibly contemporary volcanic activity at the time of the Knife Lake 

 slates. 



Finally, there were four great periods of orogenic movements, 

 denudation, and metamorphism : Following the Archean series; following 

 the Lower Huronian; following the Upper Huronian; and following the 

 Keweenawan. 



Also there were three other great periods of denudation: The Cam- 

 brian and the Cretaceous periods of base-leveling, and finally the period 

 following the Cretaceous, extending to the present time. 



