GENERAL GEOLOGICAL STRUCTURE. 457 



CHAPTER II. 



GENEEAL GEOLOGICAL STRUCTUEE OF CENTRAL 

 WISCONSIN. 



The region of country included within the boundaries of the state 

 of Wisconsin is quite simple as to the grand features of its rock 

 structure, and may be briefly described as consisting of a great nucle- 

 us of ancient crystalline rocks, encircling which, but more especially 

 on the east, south, and west, are succeeding bands of limestone and 

 sandstone, belonging to the Silurian series. 



Forming most of the northern half of the state is a great mass of 

 crystalline rocks — granites, gneisses, chloritic micaceous and horn- 

 blendic schists being the predominating kinds — which are folded 

 and eroded so as to offer the greatest obstacles to their detailed study, 

 arid which appear, for the most part, to be referable to the Laurentian 

 division of the Archaean. 



On the northern edge of this central nucleus, just south of Lake 

 Superior, in Bayfield and Ashland counties, is a narrow belt of quartz- 

 ites, magnetic and specular iron ores, diorites, talcose chloritic and 

 black slates, etc., which overlie unconformably the gneisses of the 

 Laurentian immediately to the south. This fact, taken together with 

 their nature and relations to the newer adjoining formations, would 

 seem to throw these beds, without any doubt, into the same catagory 

 with the Iron Bearing series of Michigan, and the Huronian system 

 of Canada. Similar rocks, with similar relations to the surrounding 

 formations, exist in Oconto county, on the northeast border of the 

 state, from where they stretch far into the Upper Peninsula of Michi- 

 gan, and include the famous iron regions of Marquette and the Me- 

 nomonee. On the south side of the Laurentian core, on Black river, 

 in Jackson county, are again similar rocks, whose Huronian age is 

 not so clearly made out. Still farther south, and within the area of 

 the Silurian formations, are projecting portions of the here buried 

 Archaean. These isolated masses are made up chiefly of quartzites 

 and dark-colored quartz-porphyries, and are scattered widely over 

 Marquette, Waushara, Green Lake, Columbia and Sauk counties, pre- 

 serving in their positions a sort of rough parallelism to the southern 



