302 THE GEOLOGY OF MINNESOTA. 



[Niagara limestone. 



The Niagara limestone. This formation has been indentified in Fillmore 

 county at but one point. It is much more enduring than the shales under- 

 lying it, but it enters on a drift-covered area, with small valleys of drainage 

 only, some distance south of the state line. The nearest important point 

 of its known outcrop is at Lime Springs, in Iowa. It differs from the Galena 

 limestone in being much lighter colored, especially when broken or pow- 

 dered. It is strongly crystalline, and often porous, but it is also, in some 

 parts, a very firm and enduring limestone. It also has a very diiferent and 

 much more abundant fossil fauna. It is separated from the Maquoketa 

 shales, at Lime Springs, by a limestone breccia of about eighteen inches. 

 Its color, in its heavier and close-textured portions, is somewhat grayish, or 

 leaden, and it is interbedded with hard shale which turns nearly white on 

 exposure. This shale, in broken pieces, makes up the larger part of the 

 breccia mentioned, and falls down the bluff in that condition, where it is 







lost in the weather, the framework of the cement only remaining, making 

 a curious open network or mesh, the partitions and threads enclosing angu- 

 lar apartments. The great bed of shale, which causes the water-power 

 here, may be 75 or 80 feet to the water, at the quarry of Mr. John Smith, 

 though near the mill it is reduced to ten or fifteen feet. Throughout the 

 most of that interval a heavy debris covers it from sight, the overlying 

 Niagara only being visible along the top of the bluff. The Niagara has a 

 dip of five or six degrees to the S. W., and passes below the Devonian lime- 

 stones which are exposed and quarried at Lime Springs station, about a 

 mile further south. The thickness of the Niagara included in that interval 

 may be 100 or 150 feet. This underlying bed of shale gives rise to springs 

 of limy water that enter the river along the bluff. 



In the S. E. J sec. 33, York, about forty rods north of the state line is a 

 very small exposure of the Niagara in the bottom of a ravine, with the 

 Devonian in the enclosing hillsides. A slight opening has been made in 

 these beds, which are very porous and light-colored, and about three inches 

 in thickness. Although no fossils were found here to identify the forma- 

 tion, the presence ot a very different rock well known as the Devonian, in 

 the hills and ridges surrounding it, as well as the strong resemblance it 

 bears to the Niagara at Lime Springs, will allow of its being regarded only 

 as the Niagara limestone. 



