GEOLOGICAL FORMATIONS. 669 



The beds mentioned under Nos. 8 and 9 change to a yellow, arena- 

 ceous limestone, which is extensively quarried for building stone at 

 Mazomanie, Black Earth and other towns in the vicinity. It is easily 

 quarried and dressed, and makes a good and durable building stone, 

 which does not undergo much change on exposure to the weather. 

 The sandstone is but little used for building stone, as it is too friable. 



The sandstone beds of the Potsdam are usually composed of a very 

 soft and friable sand, frequently crumbling readily between the fin- 

 gers, especially if it is white. The upper beds are more frequently 

 white than the others, although white beds are not uncommon in all 

 parts of the formation. In general, the lower beds are yellow or 

 brownish in color. The great inequality in the hardness of the sev- 

 eral strata of the Potsdam is frequently the cause of the formation of 

 terraces by erosion, which are often a conspicuous feature of the 

 valleys in the vicinity of the Wisconsin river. The following section 

 of a hill near Lumberville, on the Wisconsin, illustrates their ap- 

 pearance. 



FIG 4. 



SKETCH OP A TERRACED HILL NEAR LUMBEKVILLE. 



1. Lower Magnesian Limestone. 2. White Potsdam Sandstone. 3. Fossiliferous Shales. 

 4 and 5. Dolomitic Beds. 6 and 7. Dark-colored Sandstone. 



South of the Wisconsin river, about the head-waters of Blue 

 Mounds creek, and Mill creek, the upper bed of the Potsdam consists 

 of a red and very ferruginous sandstone, often containing seams of 

 iron ore, and iron concretions. The coloring matter appears to be 

 partially soluble, and, becoming washed out by the rain, colors the 

 soil in some places to a considerable extent. It is quite conspicuous 

 at Mr. Ruggles' farm, on the road from Dodgeville to Arena. It 



