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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, making 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 adjacent 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 following 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-