306 GEOLOGY OF EASTERN WISCONSIN. 



crystallized granular form, while the silieious and aluminous mater- 

 ial has an earthy texture, and when abundant, gives the rock a shaly 

 structure. These earthy ingredients are usually distributed in seams 

 or partings between the layers, and in thin leaves in the mass of the bed. 

 These being insoluble are left as a clay when the lime and magnesia are 

 dissolved away, giving rise to a clayey soil, or crevice filling according 

 to circumstances. When the rock is ground up by glacial action more 

 of the soluble parts are retained and a most excellent marly clay soil 

 results. In its more manifest characters and typical form, this de- 

 posit may be described as a heavy bedded, irregular, coarse textured, 

 gray or buff dolomite, containing frequent cavities lined or filled with 

 the minerals already mentioned, and weathering in a very irregular, 

 fantastic way, owing to inequalities of structure. Nodules, and occa- 

 sionally continuous sheets of chert or flint are a prominent feature 

 of some portions of the formation. When exposed at or near the 

 surface, the rock usually presents a decayed, rotten appearance. 



As a general description, this is applicable in eastern Wisconsin as 

 far north as Dodge county. At that point the formation begins to 

 undergo a change. The modification consists mainly in the intro- 

 duction of more clayey material in the form of shaly leaves and part- 

 ings. The effect of this has been to render the rock more impervious 

 to water and atmospheric agencies, and hence, its original blue or 

 gray color is more generally preserved, and to this is added the green- 

 ish or bluish hue of the shaly material, so that the rock, instead of 

 being light yellowish gray or buff, is usually greenish or bluish gray. 

 W^ith the increase of argillaceous material there is also an increase of 

 fossils. This may be partly due to the more perfect preservation that 

 was afforded by the nature of the rock, but it is probable that the 

 change in the oceanic conditions that caused the increase of clayey 

 material also had its effect upon the life of the period. 



Without entering into a full discussion of the causes that produced, 

 this modification in rock and fauna, it may be observed that the ty- 

 pical Galena limestone, viewed as a whole, arches over the low broad 

 anticlinal axis, which stretches southward from the more ancieiit 

 rocks that form the elevated country in the northern part of the state, 

 and that, whatever may be true of the western horn of this arched 

 crescent, as it enters the trough between the Wisconsin and Minne- 

 sota axes, the eastern horn becomes depressed and modified as it 

 reaches the margin of the great basin occupied by the Lower Penin- 

 sula of Michigan and adjacent regions. It will be subsequently 

 shown that there is, and was at the time of deposit, a marked depres- 

 sion of all the formations in this region, and that they were all mod- 



