GEOLOGY OF CENTRAL WISCONSIN. 



on solution, an argillaceous residue, differing in this regard from ihe 

 limy layers of the Potsdam proper, which always yield a residue of 

 white quartz-sand, with or without glauconite grains. The close 

 earthy texture of the Mendota also contrasts greatly with the porous 

 and highly crystalline character of the lower calcareous seams. 



The Madison beds, in the country about Madison, are about 35 feet 

 thick, and consist usually of pure white, frequently loose, sand, over- 

 laid by brown and yellow, firmer rock. The upper layers show gen- 

 erally a slight calcareous admixture, which locally increases to 10 or 

 15 per cent., -the rock then becoming a good building material, and 

 not being very sharply defined from the limestone above. The calca- 

 reous layers show well in the quarry just west of the city of Madison, 

 where they are as much as 15 feet in thickness, and also in the large 

 quarry near the village of Middleton. The section at the latter place, 

 given in detail on a subsequent page, is of interest as showing the 

 gradation of the Potsdam series into the Lower Magnesian, there be- 

 ing a number of thin alternating sandstone and limestone layers, 

 whilst the upper Madison beds contain as much as 50 per cent, of 

 lime and magnesia carbonates. West of Lake Kegonsa, in the town 

 of Dunn, the Madison sandstone is as much as 50 feet in thickness, 

 closely resembling the St. Peters, and grading downwards into the 

 Mendota. 



About the village of Lodi, in Columbia county, both Madison and 

 Mendota are frequently exposed, with characters like those just de- 

 scribed. Further northeast, along the western edge of the Lower 

 Magnesian, they undergo some change. At Rio the lower portions 

 of the Madison are composed of a firm, white, purely silicious mate- 

 rial, which is made up of sharply angular quartz, whilst above, the 

 ordinary brown, fine-grained rock comes in. Near Cambria, still 

 farther north, the same thing is to be seen, the Mendota layers be- 

 coming at the same time largely replaced by reddish clay-shale, but 

 still retaining in parts the typical yellow appearance. Near Mar- 

 quette, in Green Lake county, the Madison has its usual brownish, 

 friable character, whilst the Mendota is largely composed of a light 

 yellow, regularly bedded, aluminous limestone, and is not well defined 

 from the sandstone below. 



Along the valley of the Wisconsin, in Sauk and Dane counties, both 

 of the layers are well marked, the Mendota having most commonly 

 the character last described. Its regular bedding makes it valuable 

 as a building stone, and it is hence frequently quarried. Near Spring 

 Green it reaches a thickness of as much as 45 feet in all, its upper 

 layers being shaly and fossiliferous. The Madison sandstone in this 



