468 GEOLOGY 



Succession of events. Reviewing the succession of events in 

 the Lake Superior region, we find (1) that the land was high enough, 

 after the Archean rocks came into existence and were deformed, 

 for its surface to suffer prolonged erosion, but that the sites of the 

 earliest sedimentation are unknown. (2) The Archean land then 

 sank or was so eroded or deformed as to permit the deposition of 

 the Lower Huronian sediments on portions of its eroded surface. 

 (3) The area of the former Archean land, together with some part 

 of the Lower Huronian system about it, was then brought into such 

 an attitude, presumably by crustal warping, that it was subject to 

 a long period of erosion, with contemporaneous sedimentation else- 

 where. During the time of the deformation, the rocks involved 

 were somewhat metamorphosed. (4) Again the land seems to 

 have sunk, allowing the sea (or at any rate, conditions for depo- 

 sition) to cover a large part of the territory which had been subject 

 to erosion, and to deposit upon its eroded surface the sediments of 

 the Middle Huronian system. (5) After this long period of sedi- 

 mentation, certain tracts seem to have emerged, exposing the land- 

 ward border of the Middle Huronian system, and all the older rocks 

 not covered by it, to erosion. The emergence of areas of Middle 

 Huronian sedimentary formations was accompanied by some de- 

 formation and metamorphism. (6) This period of erosion was 

 followed in turn by another period of submergence, when sediments 

 (the Animikean) were laid down again in the Lake Superior region, 

 this time on the eroded surface of the Middle Huronian or some 

 older system. (7) Deformation, accompanied by emergence and 

 followed by erosion, succeeded this third period of Proterozoic 

 sedimentation. (8) Flows of lava of great magnitude were then 

 poured out upon the surface of the land over considerable areas, 

 and intruded into existing terranes. Before the outflows ceased, 

 sedimentation began again in the region, and soon predominated, 

 the lavas and sediments making the Keweenawan system. (9) 

 After the deposition of this system, much of it was exposed to 

 erosion. 



This succession of events implies repeated changes of relative 

 level of land and sea in the Lake Superior region during the Pro- 

 terozoic era. We shall see that such changes are confined neither 



