THE EARTH BEFORE THE CAMBRIAN 



393 



The Proterozoic Era 



Archaeozoic and Proterozoic Contrasted. — The Archaeozoic sys- 

 tems are separated from the overlying Proterozoic by a great and 

 widespread unconformity (p. 270) upon which rests a series of rocks 

 of enormous thickness, which extend to the fossiliferous Cambrian. 

 The two groups differ in a number of particulars. "The Archaean 

 [Archaeozoic] is a group dominantly composed of igneous rocks, largely 

 volcanic, and for extensive areas submarine. Sediments are subor- 

 dinate. The Algonkian [Proterozoic] is a series of rocks which is 

 mainly sedimentary. Volcanic rocks are subordinate. The Algon- 

 kian [Proterozoic] sediments, where not too greatly metamorphosed, 

 are similar in all essential respects to those which occur in the Paleo- 

 zoic and later periods. When the Algonkian [Proterozoic] rocks were 

 laid down essentially the present conditions prevailed on earth. The 

 Archaean [Archaeozoic] rocks, on the other hand, indicate that during 

 this era the dominant agencies were igneous. On the whole, the def- 

 ormation and metamorphism of the Archaean [Archaeozoic] are much 

 farther advanced than the Algonkian [Proterozoic]. The two groups 

 are commonly separated by an unconformity which at many localities 

 is of a kind indicating that the physical break was of the first order of 

 importance." (Van Hise.) 



The Proterozoic in Different Regions. — Lake Superior Region. 

 (Table, p. 388.) South of the " Pre-Cambrian shield " (Fig. 367) the 



Fig. 367 



JRW ./Rk" Ml 



Section through a portion of northern Minnesota, showing the relation of 

 the Pre-Cambrian rocks. (U. S. Geol. Surv.) 



r, • Akm Keweenawan. 



Proterozoic m m 



Mi 

 /Rk 



Archaeozoic 



Huronian. 



Granites and gneisses of the Laurentian. 



Schists and iron-bearing formations of the Keewatin. 



lowest member of the great series which constitutes the Proterozoic, 

 is the Lower Proterozoic (Huronian, named from the fine develop- 

 ment north of* Lake Huron), and is composed of quartzites, slate, 

 schists, interbedded lava flows, and igneous intrusions, together with 

 limestone and beds of iron ore. The rocks are usually much folded 

 and occur in the form of long, narrow belts, separated by the Archaeo- 

 zoic schists and gneisses, being small remnants of a once extensive 

 system. Locally, at least, the Lower Proterozoic (Huronian) is 



