THE HURONIAN SYSTEMS 



.I \ViMMiisjn i^ proluMy continuous beneath younger formations with the Pro- 

 tcro/.oie of soul Invest crn Minnesota, the Black Hills, and the Rocky Mountains on 

 the \vci-t, and with that of Missouri and Texas on the south. 





Fig. 304. Diagram showing how Proterozoic rock (Al) may fail to outcrop 

 about Archean (JR). 



Fig. 305. Diagram showing how Proterozoic rock (At) may outcrop on one 

 side of an area of Archean (JR) and not on the other. 



THE PROTEROZOIC OF THE LAKE SUPERIOR REGION l 



The Proterozoic formations have been most carefully studied 

 and their relations are best understood in the region about Lake 

 Superior, and the formations of this region have become, in some 

 measure, the standard of comparison for the Proterozoic group as a 

 whole. The four great unconformable systems, their relations to 

 one another, to the Archean below, and to the Cambrian above, 

 are as follows: 2 



Earliest Paleozoic 



Proterozoic 



Arc heozoic 



Cambrian 



Unconformity 

 4. Keweenawan 



Unconformity 

 3. Upper Huronian (or Animikean) 



/ 'nconformity 

 2. Middle Huronian 



Unconformity 

 i. Lower Huronian ^x^ 



Unconformity 



An hean 



The Huronian Systems 



The first three systems of the Proterozoic group have much in 

 common. All are dominantly sedimentary, and each includes 

 formations of the common sorts of clastic rock or their metamor- 

 phosed equivalents, together with limestone and beds of iron ore. 



1 Van Hise and Leith. Mono. LII, U. S. Geol. Surv. 



2 Jour. Geol. XIII, p. 161. 



