GEOLOGIC HISTORY. 445 



These iron silicates comprise chrysolites, pyroxenes, and amphiboles. 

 They are very coarse grained and they stand as the extreme of deep-seated 

 static metamorphism of an iron-bearing carbonate. The muds and grits of 

 the Knife Lake formation and the conglomerates of the Ogishke formation 

 adjacent to the gabbro were completely crystallized largely into g-ranitic- 

 textured rocks described (pp. 315, 342). These are the best representatives 

 of the production of granitic textured rocks from heterogeneous mechanical 

 sediments known to the writer. They were produced under deep-seated 

 static conditions where high temperature prevailed. 



All of the complex events thus far described preceded Cambrian time. 

 This history is Archean and Algonkian. In another place it has been 

 shown by Walcott" that the Cambrian transgression over the North 

 American continent began at the southeast and extended to the northwest, 

 and that it continued through Lower and Middle Cambrian time before the 

 sediments were deposited in the Lake Superior region. This great erosion 

 period was probably partly contemporaneous with the tilting which pro- 

 duced the Lake Superior syncline. The erosion of this time laid bare' the 

 great laccolith of gabbro, as well as the beds of the overlying lavas, and 

 thus exposed to light of day all of the great series of rocks — within the 

 Vermilion district itself the Archean, the Lower Huronian, and the Upper 

 Huronian series; south of these the great batholith of gabbro; and south 

 of this the Keweenawan lavas. Finally, however, the sea overrode this 

 region, and the Cambrian sandstone was laid down in the Lake Superior 

 Basin and along its border. Remnants of it have been found far inland, 

 but none in the Vermilion district itself However, it can not be doubted 

 that over the Vermilion district were deposited Cambrian, or Silurian rocks, 

 and therefore the Paleozoic was there represented. 



The next great step in the history of the region was the elevation of 

 the land above the sea and long- continued denudation. This period of 

 denudation has generally been known as the Cretaceous period of base- 

 leveling. Whether the sea actually overrode the Vermilion district and 

 there deposited the Cretaceous rocks is uncertain. However, it is certain 

 that the Cretaceous sea reached within a comparatively short distance of 

 the district, for outliers of the Cretaceous rocks are now known within 20 



aCorrelation Papers: Cambrian, by C. D. Walcott: Bull. U. S. Geol. Survey No. 81, 1891, p. 364. 



