THE PROTEROZOIC ERA 471 



Sequence of events elsewhere. A series of events consonant but 

 not necessarily identical with those of the Lake Superior region was 

 probably in progress about every other area of Archean rock, during 

 the Proterozoic era; but it does not follow that about every other 

 Archean land area four great systems of rocks were laid down 

 during this long era. About some such areas there may well have 

 been three, or two, systems of Proterozoic rocks instead of four, 

 while about others, continuous sedimentation may have been in 

 progress from the beginning of the Huronian period to the end of 

 the Keweenawan. 



Proterozoic Rocks in Other Regions 



Pre-Cambrian sedimentary formations occur in many other 

 parts of North America, in relations to the Archean similar to those 

 already described. On the whole, it may be said that they resemble 

 the rocks of the Proterozoic systems about Lake Superior as closely 

 as could be expected under the general principles already set forth. 

 Some of the more important occurrences of Proterozoic rocks 

 outside the Lake Superior region are the following: (1) In an exten- 

 sive area north of the Great Lakes; (2) in the eastern provinces of 

 Canada; (3) in the Adirondacks; (4) in isolated patches in the 

 Mississippi basin, in Wisconsin, northwestern Iowa and adjacent 

 parts of Minnesota and South Dakota, in the Black Hills of South 

 Dakota, in southeastern Missouri, and in Oklahoma; (5) in Texas; 

 (6) in the Piedmont belt of the eastern part of the United States; 

 and (7) at various points in the Cordilleras. 



In some of these localities, the rocks are chiefly sedimentary 

 or meta-sedimentary, while in others they are partly or even 

 largely igneous. Thus in the Black Hills, the Proterozoic rocks 

 consist of closely folded meta-sedimentary rocks, such as mica 

 slates, quartzites, schists, etc., and intrusive igneous rocks. From 

 the granite intrusions, the largest of which is eight or ten miles 



The unshaded areas north of Mexico are believed to have been land 

 during the early portion of the Cambrian period. The unshaded area 

 south of the United States represents lack of knowledge. The shading 

 within the area of the ocean is the same as in Fig. 341. The Middle 

 Cambrian may be somewhat more extensive than the map shows. The 

 area not covered by the early Cambrian formations, and probably a 

 still larger area, was land at the end of the Proterozoic. 



