THE PROTEROZOIC ERA 473 



which were formerly classed as Archean. Their position is shown 

 in Fig. 341. This belt of metamorphic rocks contains formations 

 that were sedimentary, and those that were igneous. Some of 

 them are probably Proterozoic, but the Proterozoic rocks here have 

 not been generally differentiated from the Archean on the one 

 hand, and from metamorphic Paleozoic rocks on the other. 



Summary. While the correspondence of the Proterozoic rocks 

 in these various regions with those of the Lake Superior region is 

 not generally very close, it may be pointed out again that close 

 correspondence is not to be expected, even if the rocks of different 

 localities were contemporaneous in origin. The phases of sedi- 

 mentation taking place about any land mass at any time are largely 

 dependent upon the height of the land, the freeness of the exposure 

 of its coasts, the climate, and the character of the formation suffer- 

 ing erosion. These various factors were as likely to be dissimilar 

 as similar about the various centers of sedimentation. Igneous 

 rocks form a not inconsiderable part of the Proterozoic systems, 

 and there is no apparent reason why igneous activities in different 

 regions should correspond either in time or in the nature of their 

 products. Even deformations of the crust, which are the basis 

 for the separation of the group into systems, need not have cor- 

 responded in different regions. It follows (1) that the number of 

 systems bounded by unconformities within the Proterozoic, may 

 not be the same in all regions; (2) that the thicknesses of the 

 various systems may vary within wide limits; (3) that there need 

 have been no close correspondence in the sorts of rock in different 

 regions at the outset; and (4) that they may have been meta- 

 morphosed unequally since their deposition. The dissimilarity of 

 the Proterozoic in different regions, as suggested by the preceding 

 sketch, was, therefore, to have been anticipated. 



Proterozoic Formations in Other Continents 



Proterozoic formations are believed to exist in all continents. 

 In more than one country where they have been studied, the pre- 

 Cambrian sedimentary rocks are thought to belong to at least two 

 systems, separated by unconformities. It is worthy of note that 

 in Sweden, as about Lake Superior, iron ore occurs in these for- 

 mations. 



