THE PROTEROZOIC ERA 457 



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

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

 a whole. The Proterozoic formations of this region are of great 

 thickness, and are divisible into four great unconformable systems, 

 the relations of which to one another, to the Archean below and 

 to the Cambrian above, are shown diagrammatically in Fig. 344. 

 Not all of the four systems are present at all points in this region 

 and Fig. 344 shows but three of them. They are now classified 

 as follows: 1 



Earliest Paleozoic Cambrian 



Unconformity 

 4. Keweenawan 

 Unconformity 



3. Upper Huronian (or Animikean) 

 Proterozoic Unconformity 



2. Middle Huronian 



Unconformity 

 t 1. Lower Huronian 



Unconformity 

 Archeozoic Archean 



The Huronian Systems 2 



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, 

 the last derived by alteration from beds of sediment which were 

 ferruginous at the outset. Since none of the limestones are known 

 to contain fossils, their organic origin cannot be affirmed. Each of 

 the three periods of sedimentation represented by the Huronian 

 systems was long, though their duration is unmeasured. Each 

 system contains much igneous rock, some of which was extruded 

 while sedimentation was in progress, and some intruded into the 

 sediments after their deposition. Locally, igneous rock is more 

 abundant than sedimentary. 



The unconformable relations of the three Huronian systems, 

 and the unconformity of the third below the Keweenawan, show 

 that after the deposition of the first, second, and third systems 



1 Jour. Geol., XIII, p. 161. 



2 The name Huronian comes from the area north of Lake Huron, where 

 these formations were first differentiated. 



