HISTORICAL SKETCH— LAKE SUPERIOR REGION. 1 27 



This great volcanic period was doubless one of unstable 

 equilibrium, the lithosphere falling here and rising there. One 

 of the final movements was the production of the Lake Superior 

 synclinal. This synclinal movement affects not only the 

 Keweenawan rocks, but the lower series, and in areas in which 

 the unconformity between the Upper Huronian-and the Kewee- 

 nawan is not great, there is such a likeness in strike and dip of 

 the two series as to suggest, at first, that the two are conform- 

 able. It is only as the contacts between them are followed for 

 some distance, and the Keweenawan is seen to be now in contact 

 with one member of the Upper Huronian, and now with another, 

 that it is perceived that between the two there is an uncon- 

 formity. 



What proportion of the Keweenawan had accumulated before 

 this Lake Superior synclinal began it is impossible to say. Pos- 

 sibly somewhere near the center of the Lake Superior basin were 

 the larger foci, from which the great extrusions of lava occurred, 

 and here a simultaneous sinking went on, such as is usual as a 

 result of the upbuilding of a mountainous mass of volcanic 

 material. This suggestion, if true, would also partly explain the 

 apparent absence of volcanic fragmental material which naturally 

 would accumulate near these foci. 



Nowhere are the Keweenawan rocks so closel_y folded as to 

 give them a schistose structure or a metamorphic character. 

 Their induration is almost wholly a process of cementation. 



The Cambrian Transgression. — At the close of Keweenawan 

 deposition the Lake Superior region was again raised above the 

 sea, and the pre-Cambrian erosion continued until the enormous 

 thickness of Keweenawan deposits was wholly truncated. What 

 must have been mighty mountains were reduced to mere stumps, 

 or to base level. Following this denudation, the sea once 

 more transgressed upon the land, and the horizontal Lake Su- 

 perior sandstone was deposited. It now occupies many of the 

 bays about Lake Superior. It once was much thicker, and per- 

 haps covered all but the highest points of land. Certainly it or 

 an overlying formation once was at least one thousand feet higher 



