OF WISCONSIN AND MINNESOTA. 



147 



Range, is a mere outlier, removed from the main axis of upheaval by a distance of 

 over two hundred miles ; that the greater portion of the intervening country, as 

 shown by the survey, is covered by an extensive deposit of drift, through which the 

 igneous rocks seldom penetrate, and when they do, never attain any great elevation 

 above the surrounding country ; that, moreover, though the dykes and ridges form 

 a prominent feature of the country over a few sections of land, these soon lose 

 themselves under swamps, accumulations of sand, gravel, and erratics. They seem, 

 in fact, but the effects of the expiring throes — the last heaving efforts of subterra- 

 nean volcanic fires, shooting in a linear direction from the shores of Lake Superior 

 to the southwest, that retained force sufficient to burst through the superincumbent 

 impediments, and inject their molten material over a circumscribed area above the 

 general drainage of the country ; and hence could hardly lie expected to yield 

 lasting and productive metallic treasures, unless by deep-seated, laborious, and 

 expensive mining operations, in which few would be willing to embark, especially 

 when these have to be undertaken in a hard and refractory rock, like the trap of 

 the St. Croix. 



SECTION IV. 



ITS RANGE, EXTENT, AND BEARING. 



Under the title of " Formations of the Interior of Wisconsin and Minnesota," I 

 purpose to comprehend all of Wisconsin lying north of the lowest Protozoic strata, 

 as far as the sources of streams emptying into Lake Superior, together with South- 

 western Minnesota, including the Valley of Red River of the North. 



GRANITIC RANGE, FALLS OF THE CHIPPEWA. 



As a whole, this part of the District may be regarded as a vast region of drift ; 

 since the area of actual exposure of igneous and metamorphic rocks is quite limited, 

 being confined chiefly to a few points in the valleys of rivers where the streams 



