338 PROTEROZOIC ERA 



Lake Superior been high enough at any one time to furnish the thick 

 sediments of the Proterozoic, their height would perhaps have sur- 

 passed any existing elevation; but it is not probable that such ele- 

 vations existed at any time. It is more probable that as erosion 

 proceeded, the land reacted by rising slowly, or that the sea bottom 

 sank, drawing off the waters and leaving the land relatively higher. 

 In this way, degradation and elevation may have been in progress 

 at the same time, and the one process may never have got far ahead 

 of the other. The doctrine that the surface of the lithosphere sinks 

 and rises under increase and decrease of load is one phase of the 

 general theory of isostasy. 



Succession of events. Reviewing the succession of events in 

 the Lake Superior region, we find (i) that land composed of Archean 

 rocks suffered prolonged erosion, but that the sites of the earliest post- 

 Archean sedimentation are unknown. (2) The land then sank or was so 

 eroded or deformed as to permit the deposition of the Lower Huro- 

 nian sediments on parts of its eroded surface. (3) Areas including 

 Archean and Lower Huronian rocks then came into such an attitude, 

 presumably by crustal warping, that they were subject to a long period 

 of erosion, with contemporaneous sedimentation elsewhere. Dur- 

 ing the deformation, the rocks involved were somewhat meta- 

 morphosed. (4) Again the land seems to have sunk, allowing the 

 sea (conditions for deposition) to cover a large part of the area 

 which had been subject to erosion just before, and to deposit upon 

 its eroded surface the sediments of the Middle Huronian system. 

 (5) After this long period of sedimentation, certain tracts seem to 

 have emerged, exposing the landward border of the Middle Huronian 

 system, and the older rocks not covered by it, to erosion. This 

 emergence of areas of Middle Huronian sedimentary formations was 

 accompanied by some deformation and metamorphism. (6) This 

 period of erosion was followed 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 consider- 

 able areas, and intruded into older terranes. Before they ceased, 

 sedimentation began again in the region, and soon predominated, 

 the lavas and sediments making the Keweenawan system. 



