108 Wisconsin Academy of Sciences, Arts, and Letteis. 



the more able and willing to do since no special preparation 

 is required, the subjects being those upon which my thoughts 

 have constantly dwelt for many months past. 



I. There are four distinct groups of rocks in northern 

 Wisconsin, the Laureniian granites, gneiss, and schists ; the 

 Huronian schists, quartzites, iron ores and diorites ; the mela- 

 phyrs, porphyries, greenstones, conglomerates, sandstones and 

 shales of the series known now as the ''copper bearing rocks;" 

 and the lower silurian sandstones. In addition to these are 

 enormous thicknesses of quarternary clays and of boulder 

 drift. 



The Laureniian (I on the map and section) rocks, are always 

 furthest removed from the lake, never coming nearer to it than 

 eighteen miles, and being usually much more distant than 

 this. 



So far as observed these Laurentian rocks are altogether 

 granitic, gneissoid or syenitic in character, though undoubt- 

 edly various schistose beds must be present, since they are 

 found just east of the eastern limit of the district, within the 

 area of the upper peninsula of Michigan. In Wisconsin the 

 rocks of this group are almost everywhere overlaid by enor- 

 mous accumulations of drift material, showing through this 

 covering in but very few places. This overlying drift is 

 heaped up in masses which sometimes attain the altitude of 

 from 1,100 to 1,200 feet above lake Superior. Amidst these 

 drift heaps, and amidst the swamps which everywhere cover 

 the country between them, the northward and southward flow- 

 ing streams iuterlosk in an intricate manner, the former in a dis- 

 tance often not more than thirty miles from their sources to their 

 mouths, falling as much as 700 or 800 feet. It can readily be 

 seen from this that their courses must be a series of chutes and 

 falls, which is the fact, the single falls reaching in many in- 

 stances a height of from 60 to 70 feet, and in one instance at least, 

 that of Black river in Douglas county, a height of 160 feet. 



