502 GEOLOGY OF CENTRAL WISCONSIN. 



gasteropods are frequently of very large size, their rough casts and 

 impressions filling entirely a two-inch layer, whilst for a number of 

 feet above and below the rock may be entirely barren. 



The economic contents of the Trenton beds are building stone and 

 limestone for flux. Certain beds of the Blue in the lead region are 

 said to be hydraulic, and the property is probably not entirely con- 

 fined to the rock in that district. The Trenton limestone is also one 

 of the layers in which the lead ore of the lead region occurs. A 

 small crevice occurs naar the base of the Trenton, in the town of 

 Fitchburg, Dane county, from which a few hundred pounds of galena 

 have been taken. For the most part, however, the Trenton is with- 

 out sign of mineral wealth until the limits of the lead region are 

 reached, in the western towns of Dane county. East of this it occurs 

 usually in such small thickness that it could not be looked to to yield 

 any amount of ore, even if it should be metalliferous, of which, how 

 ever, there is no indication. 



The Buff limestone is used for building everywhere where it occurs. 

 It can be obtained in quite even blocks and slabs of suitable thickness 

 both for building and paving, presenting, when laid in wall, a uni- 

 form straw color. The thinner layers are also frequently used for 

 stone fences. 



The application of the blue or non-magnesian limestone layers as a 

 flux in iron smelting is certainly worthy of attention. For most of 

 the furnaces in Wisconsin and the northern peninsula of Michigan, 

 limestone is brought all the way from Kelley's Island, in Lake Erie, 

 whilst others use unsatisfactory native dolomites. The Kelley's 

 Island rock contains much more magnesia (15-20 per cent.) than the 

 Blue limestone, but is otherwise often purer, carrying almost no 

 earthy or silicious impurities. It is without doubt this purity that 

 makes it prized for smelting the hard silicious ores of Lake Superior. 

 All of the silica, however, in the Blue limestone is in the state of clay, 

 whilst in freedom from magnesia it ranks far above the Kelley's Island 

 stone, and moreover, as shown by the third of the analyses above giv- 

 en, it is at times free also from the earthy impurities. 



THE GALENA LIMESTONE. 



This formation is found in the Central Wisconsin district only in a 

 few small cappings in the tovvn of Christiana, eastern Dane county, 

 and on the top of some of the narrow ridges of the towns of Spring- 

 dale and Primrose, on the west side of Dane county. Since it is so 

 unimportant, and at the same time plays so large a part in both the 



