THE PROTEROZOIC ERA. 203 



rock, not in surface association with the Archean, may have arisen. 

 (1) If after the deposition of the Proterozoic systems the strata 

 were folded, certain portions might have been brought above sea level, 

 constituting land. If such areas were never buried, the present ex- 

 posure of the Proterozoic rocks would be explained. (2) Again, 

 during the long period of erosion which succeeded the uplift following 

 the time of Proterozoic sedimentation, the surface may have had its 

 topography greatly changed. If it were brought to the condition of 

 a peneplain with pronounced monadnocks standing upon it, as in 

 south-central Wisconsin, later sedimentation, accomplished during a 

 subsequent period of submergence, may have failed to bury these 

 elevations. Isolated outcrops of Proterozoic might represent such 

 unreduced areas. (3) Or, still again, if the monadnocks were buried, 

 erosion may have since removed the beds which were deposited over 

 them. (4) Uplift and erosion of any surface beneath which the Algon- 

 kian rocks lay, would have exposed them without exposing the Archean, 

 if erosion were sufficiently deep to remove all overlying beds, and not 

 so deep as to reach the Archean below. 



The following general statements concerning the Proterozoic of 

 different localities give some idea of its extent and relations: 



North of Lake Huron.— The Proterozoic formations of this region have already 

 been referred to (p. 181). The Keweenawan series does not seem to be repre- 

 sented, though it appears but a short distance to the west; but its absence vio- 

 lates no principles which have been enunciated. If lava here failed of extrusion, 

 and if the region was not in the attitude necessary for sedimentation during the 

 Keweenawan period, the corresponding system of rocks should be absent. The 

 strata of this area are much less folded and metamorphosed than the correspond- 

 ing formations in many parts of the Lake Superior region, showing that dynamic 

 action was not equally severe in different regions, and that the degree of metamor- 

 phism is not a safe criterion by which to determine age. 



The great northern area. — In the extensive area north of the Great Lakes, 

 Proterozoic rocks are known to have wide distribution, and to be associated with 

 still larger areas of Archean. The former often occupy' belts (troughs) in the 

 latter (see Fig. 65) and their separation is often a matter of difficulty. At some 

 points about Hudson bay, there appear to be three systems of rocks between the 

 Archean and Cambrian, though it cannot be asserted that these systems cor- 

 respond with those farther south, and the corresponding names are not commonly 

 applied to them. The several series appear to be separated from one another 

 by unconformities, and the lowest series is believed to be unconformable on 

 the Archean. 



