GENERAL GEOLOGICAL STRUCTURE. 459 



the Upper Peninsula of Michigan, and enter "Wisconsin on its north- 

 eastern corner with a sharp southwestward trend, having thus accom- 

 modated themselves to the southern line of the ancient Archaean con- 

 tinent. On crossing into Wisconsin, these formations dip quite rap- 

 idly to the eastvFard, and their southward trending outcrops succeed 

 one another in comparatively narrow bands. As they are traced 

 southward, however, these outcrops curve gradually westward, the 

 eastward dip at the same time lessening, and the exposed portion of 

 each formation becoming wider. Along the central north and south 

 axis of the state, the eastward dip has entirely disappeared, each form- 

 ation is the surface rock over a wide extent of country, and the bound- 

 ing line of each makes a wide bow to the southward before the return 

 northwestward, parallel to the western side of the Archaean area. 

 Thus it follows that in the Central Wisconsin district are to be ob- 

 served a smaller number of the Silurian formations than occur further 

 eastward and westward in the state. 



The latest one of the Silurian formations of Central Wisconsin is 

 of the age of the Trenton limestone of ISTew York. Elsewhere in 

 the state, the Upper Silurian is represented by immense thicknesses of 

 limestone, and over a small area near Milwaukee, even Devonian beds 

 are to be seen. The exact extent to which the original areas of these 

 various formations exceeded their present ones, it is quite difficult to 

 arrive at, so great has been the amount of denudation. 



After the close of the Silurian — for much of Central Wisconsin 

 probably after the close of the Lower Silurian — no farther deposi- 

 tions of any kind were made until the time of the Glacial Drift, 

 when immense masses of gravel and boulders, as also stratified sands 

 and clays, were largely deposited. During all of the intervening 

 time the region must have been out of water and exposed only to the 

 ordinary subterial eroding agencies. Thus we see how it is that we 

 find here proofs of a denudation unusually great for non-mountainous 

 regions. 



The following table shows at a glance the several formations that 

 enter into the structure of the Central Wisconsin district, with their 

 geological relations: — 



