OF WISCONSIN AND MINNESOTA. 



151 



SECTION VI. 



ITS LOCAL DETAILS. 



Crystalline Ranges of Wisconsin. — The most southwesterly exposure of granitic 

 rocks on the tributaries of the Upper Mississippi, is on Black River.* The locality 

 of their first outburst is on the northeast portion of Township 21 north, Range 4 

 west, of the 4th Principal Meridian. The predominating rock is pure granite, 

 associated with a gray syenitic rock. An extensive sand prairie stretches away to 

 the south of this range, formed by the debris of the lower Protozoic sandstones, and 

 having much the appearance of the great sand plains on the Wisconsin River, south 

 of Whitney's Rapids, represented by the topography on Section No. 3, S. 



North of the granitic range, a bed of greenstone trap, about thirty feet wide, 

 crosses the bed of Black River, and is exposed a few feet above the water-level. 

 In contact with this, are green and red chloritic, schistose rocks on Township 21 

 north, Range 4 west, of 4th Principal Meridian ; some of the latter is highly 

 ferruginous, and passes gradually into heavy beds of brown oxide and magnetic 

 iron ore. Here the magnetic needle has a considerable local variation over an area 

 of several miles square. 



The magnesiau slates, above mentioned, show themselves on the river for the 

 distance of nearly a mile, capped unconformably by a pebbly sandstone, F. 1, a, as 

 shown on Section No. 1, R, constructed from observations by Mr. Randall. Though 

 the soil of this township is sandy, from drift materials derived from the adjacent 

 sandstones, still it is of much better quality than that further south, and southwest, 

 where the Protozoic sandstones of Black River alone occupy the surface. It sup- 

 ports a growth of yellow pine. 



Four to six miles northeast of the Falls of Black River, in Townships 21 and 22 

 north, Range 3 west, of the 4th Principal Meridian, there are outliers of the above 

 sandstone formation, forming hills of seventy to one hundred feet in height ; on 

 Sections 4 and 33, 5 and 32, 1 and 36, of the above townships, some of its beds are 

 impregnated with iron to such an extent as to render it impracticable to survey the 

 country with any degree of accuracy with a common magnetic needle : the variation 

 along the line between these townships is in some places even as great as 13° 35'. 



After passing the east fork of Black River, the crystalline rocks extend, with few 

 exceptions, nearly to the tops of the highest hills, with only here and there a cap- 

 ping of pebbly sandstone. Through this granitic range, the soil is much improved, 

 and the land well timbered with hard wood and white pine of superior quality. 



Immediately after crossing Cunningham's Creek, a gneiss formation takes the 

 place of the granite, and extends some five or six miles. 



* The late Mr. J. S. Thayer observed a locality of granite in Dodge County, Wisconsin, on the west 

 branch of Hock River, on Section 33, Township 9 north, Range 13 west, of 4th Principal Meridian. 

 This is nearly on a parallel of latitude with Painted Rock, on the Mississippi, and about one hundred 

 miles east of these granitic ranges, on Black River, and distant at least one hundred and twenty miles 

 from the Mississippi, and between fifty and sixty west from Lake Michigan. 



