GEOLOGICAL EXPLORATIONS AND LITEEATUEE. 45 



statement of certain results, further noted below, in connection with the 

 final report of the Wisconsin Survey. 



1880. 



Irving {11. I).). Tlie Geological Structure of Northern Wisconsin. In the 

 Geology of Wiscousin, vol. iii, pt. i, pp. 1-25, with outline map and plate of 

 sections. Madison, 1880. 



As the title indicates, this portion of the final report of the Wisconsin 

 Survey gives a general summary of the conclusions reached as to northern 

 Wisconsin by the various members of the survey corps who had worked 

 in this region, including the authoi*. The following quotation shows the 

 more general conclusions reached as to the two formations, gneissic and 

 slaty, with which we are particularly concerned in the present volume: 



In a former volume of tliis report, I have shown how the Silurian limestone and 

 sandstone formations of the southern, eastern, and western portions of Wisconsin 

 curve concentrically around three sides of the Laurentian nucleus of the northern 

 jiart of the state. On the northern or lake Superior side, however, we tind an alto 

 gether different structure; and it is evident at once that the Laurentian nucleus has 

 constituted a barrier between the lake Superior and Mississippi valley regions since 

 pre-Silurian times. 



Laurentian system. — The rocks of the crystalline nucleus itself are referred to 

 the Laurentian of Canada, because (1) they sustain precisely the same structiu\il rela- 

 tions to the Hiuonian, Keweenawan, and Lower Silurian, as observed in the case of 

 the typical Laurentian of Canada, and (2) because they have the same general litho 

 logical peculiarities that characterize the Canada series. There can, indeed, be no 

 reasonable doubt that they are directly continuous with the Canada Laurentian. 

 Tiiey extend to the shores of lake Suj^erior in the vicinity of Marquette, Michigan, 

 and appear again on the eastern or Canada shore of the lake. The separation be 

 tween the Wisconsin Laurentian and that of Canada is therefore only a superficial one, 

 the connection l)eing concealed by the waters of tlic lake, and by the overlying Silu- 

 rian depositions in the eastern extension of the upper i)eninsula of Michigan. 



In Wisconsin, the northern limit of the Laurentiitn approaches most nearly to 

 lake Superior on the Montreal river, which is here the state boundarv — the distance to 

 the lake shore in a direct line being only 13 miles. From the Montreal river the noith 

 ern limit trends about southwest by west, and on Bad river, 25 miles farther west, it is 

 25 miles from the lake. From Bad river the course is in general but little south of 

 west, to Numakagon lake, T. 44, E. C W. Here a rapid change to a more southerly 

 direction comes in, and we find ourselves following the western side of the Laurentian 

 nucleus, soon to be bounded by the regular Mississippi valley formations. The south- 



