THE GEOLOGY OF MINNESOTA. 



[Devonian limestones. 



any desired dimension than cut or broken, since it fractures treacherously; 

 yet it is not in the least crystalline. Although a sandstone it contains no 

 apparent grit, and is useful for fine whetstones, or for hone-stones. As seen 

 about the city it is very generally of a dirty buff color to the depth of half 

 an inch or even three inches, depending on the amount of weathering or 

 oxidation. The thinner beds are altogether changed to that color. In the 

 center of the beds, however, in the deeper parts of the quarry, the stone is 

 blue. The presence of occasional concretionary iron-and-mud balls causes 

 a rusty stain ef a yellow color over the surface of many of the slabs. These 

 concretionary balls fall out, or dissolve out when in the water, and leave 

 cavities that become larger still. Some other cavities that have been pro- 

 tected within the homogeneous rock, on fracturing the rock are seen lined 

 with drusy quartz, and the quartz is sometimes coated with a limonite 

 scale. The rock contains very sparingly a few molluscous fossils. These 

 are generally too much absorbed, or too fragmentary, to admit of specific 

 identification. Among these Prof. H. S. Williams has made the- following 

 determinations, which, however, are to be regarded "as nothing more than 

 strongly probable," viz: numerous cavities of Aulvjivni. or some allied form, 

 a small shell like Atrypa reticularis, and another like Atrypn nxp't-a, H., 

 Cyrtina, like C. Dalmani, but perhaps C. Hamiltonensis, several lenticular- 

 shaped shells wflich are probably Nucleospira, a minute terebra,tuloid shell 

 of Rensselaeria type, trace of a crinoid stem, and a trace of a minute Ortho- 

 ceras. At Gregson's mill he has also identified the following, viz: Produc- 

 tella truncata, Hall, a minute lamellibranch like a small Arintlopecten, and a 

 minute brachiopod of an oval, smooth surface resembling a dorsal valve or 

 Ambocoelia or (?) Nucleospira. On the strength of these Prof. Williams is 

 of the opinion that the fauna belongs to an horizon near the base of the 

 Hamilton, either below it or in an equivalent position to the New York 

 Marcellus. 



At the mill of J. Gregson, about two miles below Austin, a great deal of stone has formerly 

 been taken out, but now the quarries of that neighborhood are nearly all flooded by water of the 

 dam. The chief quarry was just above the present site of the mill and near the dam, on the 

 left side, though just below the dam the rock shows on both sides and has also been wrought. 

 The following downward section was seen at this point. 



Section at Gregson's mill. 



No. 1. Black loamy soil 7 to 8 feet. 



No. 2. Loose fragments of the underlying beds, and clay, mixed 3 feet. 



