HISTORICAL SKETCH— LAKE SUPERIOR REGION. 123 



Bay and north of Lake Superior. How far the sea transgressed 

 over this area, and whether it also advanced toward it from the 

 north and west, is unknown. It is probable as the sea advanced 

 from the south, that the great mass of fragmental detritus, mak- 

 ing up the Baraboo and Sioux quartzites, was laid down before 

 the sea had transgressed to what is now the north shore of Lake 

 Superior, and thus would be explained the discrepancy in the 

 parallelism of formation between the Sioux quartzites, Baraboo 

 quartzites, etc., and the districts of Upper Huronian rocks adja- 

 cent to Lake Superior. 



In this case the advancing ocean was perhaps making its 

 progress by cutting a terrace quite as much as by subsidence. 

 However, there is reason to believe that the area included within 

 the west end of the Lake Superior Basin, i. e., from the Animikie 

 series to the Mesabi range, and thence to the Penokee series was 

 submerged practically at the same time. For here we have three 

 great formations of like character in identical order. The lowest 

 formation, the quartzite and quartz-slate with conglomerates 

 derived from the Basement Complex and the Lower Huronian, 

 are the first deposit of the advancing sea. After this came a 

 deepening of the water, when the calcareous and ferruginous 

 formation, now constituting the iron-bearing member, was laid 

 down. Then perhaps as a consequence of the upbuilding of this 

 formation, came a shallowing of the water and the deposition of 

 the great thickness of clayey sediments of the Upper Huronian. 

 Since the last formation must have been deposited in shallow 

 water, and yet is of great thickness, the bed of the ocean was 

 probably subsiding during the remainder of Upper Huronian 

 time. 



At the end of the deposition of the Upper Huronian rocks, 

 the Lake Superior region rose above the sea, and the atmospheric 

 forces once more set to work. The orographic movement follow- 

 ing the Upper Huronian, like that following the Lower Huronian, 

 was locally intense, but in general the folding was of a gentle 

 character. Along narrow axes the plications were so severe as 

 to give the Upper Huronian rocks a foliated structure and com- 



