446 THE VERMILION IRON-BEARING DISTRICT. 



miles southwest of Vermilion Lake, and it seems highly probable that Cre- 

 taceous rocks were laid over the disti'ict. However, in this case the long- 

 continued erosion of Cretaceous time had probably removed all of the 

 Paleozoic sediments, and had reduced the land to a rough peneplain before 

 the deposition of the Upper Cretaceous rocks. 



It can not be said that this period of base-leveling in the Vermilion 

 district was nearly so complete as in central Wisconsin. However, the 

 hills rise to approximately the same altitude. If one ascends to some high 

 point he finds an approximate horizon line above which onlj^ a few points 

 project, as, for instance, the Sawtooth range. 



Following the period of Cretaceous base-leveling the land was raised 

 approximately to its present altitude, and a second cycle of erosion was 

 inaugurated. This cycle has continued to the present time. In the early 

 part of this long cycle river erosion was the impoi'tant factor, and at this 

 time were scooped out the longitudinal valleys following the softer rocks 

 and the various structures of the rocks. The drainage was adjusted to the 

 character of the rocks. The slate areas were largely valleys; the more 

 resistant areas were largely highlands. Finally, there came the various ice 

 advances, at which time the valleys were widened and deepened, the hills 

 were rounded, and glacial debris was dropped here and there, but especially 

 in the valleys. When the ice last receded the present topography was sub- 

 stantially shaped. The depressions filled with water until they overran 

 their rims at the lowest points, thus forming the lakes. The lakes were 

 thus connected by streams, and the present irregular drainage (discussed 

 on pp. 39-46) was inaugurated. Thus we have explained the present 

 topography of the district. 



In conclusion, we see that the Vermilion district presents one of the 

 longest geologic histories of any region in the world. Apparently the Ely 

 greenstone, the most ancient formation, was laid down in primeval time 

 and the Soudan formation was deposited above it. Since that time there 

 were five great periods of deposition: The Lower Huronian, the Upper 

 Huronian, the Keweenawan, the Paleozoic, and the Cretaceous. 



There were four great periods of igneous activity: The Ely greenstone, 

 the great batholithic intrusions at the end of Archean time, the hardly less 

 important batholithic intrusions at the end of Lower Huronian time, and the 

 great Keweenawan period of volcanic extrusion and intrusion. There was 



