340 PROTEROZOIC ERA 



Archean land area four great systems of rocks were laid down dur- 

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

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

 while about others, continuous sedimentation may have been in 

 progress from the first of the Huronian periods 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, they resemble the rocks of the 

 Proterozoic systems about Lake Superior as closely as could be ex- 

 pected under the general principles set forth. 



Some of the more important occurrences of Proterozoic rocks 

 outside the Lake Superior region are the following: (i) 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 Mis- 

 sissippi 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 (Fig. 295). 



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 

 slates, quartzites, schists, etc., intruded by granite. From the 

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

 and nearly as broad, numerous dikes penetrate the clastic beds, and 

 furnish good illustrations of the metamorphosing effects of igneous 

 intrusions. In the Adirondack region, pre-Cambrian rocks make 

 up the larger part of the mountain mass. They include both sedi- 

 mentary (meta-sedimentary) and igneous rocks, the latter partly 

 at least intrusive in the former. 



The Cordilleran region. The cores of many of the older moun- 

 tain ranges of the west are believed to be of Archean rock. In 

 many of them there are thick series of sedimentary or meta-sedi- 

 mentary rocks (Proterozoic) overlying the Archean and surround- 

 ing its outcrops, overlain in turn by Cambrian or younger strata. 

 Sedimentary formations predominate among these Proterozoic 

 formations, but are associated with igneous rocks which are in part 



