298 THE GEOLOGY OF MINNESOTA. 



[Galena limestone 



ventional aspects of the Galena. The rock shows a slight dip to the south. 

 Mr. Andrews has built a stone barn and stable. 



The only separating horizon between the " Upper Trenton" and Galena 

 limestones is a lithological change in the rock. There is no unconform- 

 ability between the layers of the formations, and there in no known differ- 

 ence of fossil contents. Near the upper portion of the calcareous strata, 

 occasional layers appear that are much more porous and have a light butt 

 color. They are also much thicker than the layers of the Upper^Trenton, 

 reaching, after the change is fully established, a thickness of four or five 

 feet. Mingled with these heavy magnesian layers are thinner layers of green 

 shale. When these heavy magnesian beds are near the top of a bluff they 

 give it a roughness but at the same time a persistence of outline which the 

 thinner beds alone do not possess. This rock is sub-crystalline. It con- 

 tains numerous cavities of irregular shape, some due to the weathering out 

 of carious material and some to the absorption of fossils. It holds consid- 

 erable masses of calcite, and sometimes lumps of galena, from which it has 

 derived its name. Although the " Galena limestone " near Dubuque, in 

 Iowa, is stated by Prof. J. D. Whitney to be about 250 feet, (Geology of Wis- 

 consin, Vol. I., p. 172), the distinctive Galena characters enter Minnesota 

 with a thickness much less than that. From all that can be seen of the 

 strata in Fillmore county, they appear to be less than seventy-five feet 

 thick. The Trenton, on the other hand, is given by the same authority, at 

 seventy feet, average thickness, at Dubuque. 



The characters that distinguish the Galena are not constant.. In Fill- 

 more county the "lead fossil", Receptaculites, pervades the strata as low as 

 the green shale, at least, although regarded as characteristic oi the Galena: 

 and the Lingula quadrata (or its near ally, with which it seems to have been 

 confounded, Lingula Elderi, Whit.,) also said by Prof. Whitney not to appear 

 in the lead region in the "blue" nor the ''buff," is found throughout both. A 

 very fine specimen was obtained, of Lingula Elderi, at Mr. Taylor's quarry 

 near Fountain, from the Trenton ("buff limestone" of Prof. Whitney), and 

 of L. Cobourgensis, Bill., from Chatfield from the same horizon. Lithologically 

 also the two limestones appear to merge into one another. The compact, 

 hard, blue limestone, characteristic of the lower beds, gives place near the 

 top to a lighter-colored, slightly vesicular, even-grained, more heavily bed- 



