558 GEOLOGY OF CENTRAL WISCONSIN. 



district. No traces of fossils have ever been observed in Central Wis- 

 consin. 



The only economic contents of the St. Peters are to be found in 

 the sand of which it is made. This can be shoveled out and used for 

 all purposes to which sand is ordinarily applied. Frequently the 

 sand is of such purity and whiteness as to be of excellent quality for 

 glassmaking, but, as already said, this phase of the formation is more 

 characteristic of its development in the western part of the state and 

 along the Mississippi. 



THE TRENTON LIMESTONE. 



In Wisconsin and the adjoining portions of Minnesota, Iowa and 

 Illinois, the St. Peters sandstone is succeeded by 300 to 350 feet of 

 limestone beds. These are apparently the equivalents of the Treaton 

 series of New York, but comprise two well marked members, the up- 

 per one of which has no exact representative among the eastern rocks, 

 whilst the lower and thinner of the two, as indicated by its numer- 

 ous fossils, represents exactly the Birdseye and Black river limstone. 

 To this lower member exclusively it has become customary in Wis- 

 consin to attach the name of Trenton, the upper being known as the 

 " Galena " limestone, from the fact that it is the main repository of the 

 lead ores of the Upper Mississippi lead region. This nomenclature is 

 retained in the present report. 



In the Central Wisconsin district the Trenton limestone has a sur- 

 face distribution of about 220 square miles, being confined wholly to 

 Dane and Columbia counties. In the latter county it occurs in two 

 principal areas, one in the northeast occupying the eastern and cen- 

 tral parts of Randolph, and the northeast part of Court-land; the 

 other, in the southeast, covering southern Columbia and southeastern 

 Hampden. In Dane county the formation has a much wider spread. 

 In the towns on the east side of the Catfish valley it covers all the 

 higher grounds, occurring in a number of detached areas of very dif- 

 ferent sizes. Some of these are quite small, running from a few acres 

 to one or two square miles in extent, as in Medina and Deerfield, 

 where they are very numerous; others, however, cover the greater 

 part of a township, or even two or three townships, as in the case of 

 the large one which occupies nearly all of Christiana and Albion, with 

 considerable portions of Pleasant Springs and Dunkirk. On the west 

 side of the Catfish, in Rutland, Oregon, Fitchburg, Verona and Mont- 

 rose, are a number of small areas of Trenton, occurring as isolated 

 ridges amidst a lower country occupied by the St. Peters. A large 



