300 THE GEOLOGY OF MINNESOTA. 



I Maquoketa shales. 



The bluffs at Lime City consist of one kind of rock, being this dolomitic 

 Galena, but rather more even and in thinner beds than is seen in some other 

 places, making good white lime. They rise about seventy-five feet, but the 

 perpendicular rock-bluffs are generally not more than forty. Some of the 

 beds v/hen quarried become three and four feet thick, and are firm and 

 crystalline. They contain the fossils Murchisonia and Maclurea, and a 

 Strophomena which is very prolonged-convex, like fluctuosa; also Endoceras 

 magniventrum, and another orthoceratite at least nine inches in diameter 

 and circular in section, and the "sun-flower coral." 



At Weisbach's the top of the bluff is magnesian, and like that burned 

 for lime at Lime City, amounting to about fifty feet, but below that the 

 rock is more nearly a pure limestone ; at least it is sometimes gray and 

 aluminous, and sometimes like the compact rock at St. Charles in Winona 

 county, ashen and brittle and fossiliferous; while in other places it is so 

 aluminous as to disintegrate like that at the railroad cut near Spring 

 Valley. There is perhaps a thickness of a hundred and twenty-five feet 

 of this gray "Upper Trenton" rock below the magnesian beds that are burnt 

 for lime. , The level of the dam at Weisbach's must be nearly on the top 

 of the green shale, but it can not be seen. 



The Maquoketa shales. In Iowa Dr. C. A. White has given this name to 

 a series of shales overlying the calcareous beds of the Trenton and Galena.* 

 These were then believed to be the sole representative of the Hudson River 

 rocks, in the Northwest, but since the underlying shales (the "green shales" 

 of this report) contain well known Hudson River fossils, the whole interval 

 of strata from the Lower Trenton to the Niagara are allied to that geologi- 

 cal epoch. While it is very probable that this upper series of shales enters 

 Minnesota from the south, being seen two and a half miles south of the state 

 line, at Lime Springs, it has, as yet^ hardly been identified within the limits 

 of the state. Being made up of soft materials its outcropping edge is apt 

 to be hid by the falling down of drift or loam, or of the overlying limestone. 

 It will probably be a long time before any well authenticated localities of 

 its'existence are known. The following points may be mentioned at which 

 possibly the upper shales exist in Fillmore county, viz., the shale excavated 

 in the mill race of De For's mill, N. E. ^ sec. 25, Bloomfield, and N. E. \ sec. 



Geology of Iowa. 1870. Vol. I., p. 180. 



