CHAPTER VI. 



LEAD AND ZINC. 



2.06.01. Example 24. ' The Upper Mississippi Valley. Gash 

 veins and horizontal cavities (flats), limited to the Galena and 

 Trenton limestones of the Upper Mississippi Valley, and contain- 

 ing galena, zincblende, and pyrite (or marcasite), with calcite, 

 barite, and residual clay. The deposits are found in southwest 

 Wisconsin, eastern Iowa, and northwestern Illinois. The greater 

 portion of the productive territory lies in Wisconsin, and covers 

 an area which would be included in a circle of sixty miles' radius, 

 whose limits would pass a few miles into Illinois and Iowa. A low 

 north and south geanticline runs through central Wisconsin dat- 

 ing back to Archaean times and called by Chamberlain "Wisconsin 

 Island." On its western slope the Cambrian and Lower Silurian 

 rocks are laid down, and these in the western limit of the lead dis- 

 trict pass in the adjoining States under the Upper Silurian. They 

 are folded also in low east and west folds, but in the aggregate 

 the whole series dips very gradually westward. The chief east 

 and west fold forms the south bank of the Wisconsin River, and 

 may have been the cause that deflected it from a southerly 

 course. The easterly part of the lead region is 350 feet higher 

 than the western, and the northern is 500 feet above the southern. 

 The general slope is thus southwesterly. 



2.06.02. The Galena limestone is a dolomite reaching 250 feet 

 in thickness. .On the hilltops left by erosion Maquekota (Hudson 

 River) shales are seen. The Galena has shaly streaks, which have 

 largely furnished the residual clay of the cavities. There are also 

 cherty layers and sandy spots. Under the Galena lies the Tren- 

 ton, from 40 to 100 feet thick, and made up of an upper blue por- 

 tion, which is a pure carbonate of lime, and a lower buff portion 

 that is magnesian. The upper portion of the blue has a band of 

 shale locally called the " Upper Pipe Clay," and the pure, crypto- 

 crvstalline limestone under this is called " Glass Rock." The blue 



