Lozi'cr Silurian of JViscoitsiii mid Minnesota. 319 



The fossils are for the most part, marked out by discolora- 

 tion (brown or red), but a few by cleavage only. They are quite 

 numerous and are easily found when one once knows how and 

 where to look for them. 



They occur fifty or more feet below the top of the formation. 



I have assigned the specimens found, to the genera and 

 species to which I think they belong. They are remarkably like 

 species found in the low^er part of the Trenton shales and in the 

 Trenton limestone which here rests conformably on the Saint 

 Peter sandstone. And it may be, as has been suggested, that the 

 Saint Peter is of the Silurian rather than that of the Cambrian 

 formation. 



As soon as spring opens, I shall spend some days in a more 

 thorough search, in order to find out as far as possible, the true 

 nature and horizon of these fossils in the Saint Peter sandstone. 



February 3, 1891. 



THE LOWER SILURIAN FORMATIONS OF WISCO^^SIN AND MINNESOTA 



COMPARED. — F. W. Sardeson. 



It is the purpose of this paper to give some observations on 

 the Silurian of Minnesota, and the Trenton group in particular; 

 and to compare it with the same of Wisconsin. 



There are some difficulties in undertaking such a comparison. 

 For example, the Trenton group in Wisconsin is nearly all lime- 

 stone, while in Minnesota it is largely composed of shales. This 

 lithological difference is accompanied by some differences in the 

 fauna and in the outward appearance of the fossils. Then, too, 

 four beds are recognized in the Trenton of Wisconsin, the Lower 

 Buff, Lower Blue, Upper Buff and Upper Blue beds, while in 

 Minnesota two are usually spoken of — Trenton limestone, or 

 shell beds, and Trenton shales, or green shales. These difficulties 

 I shall aim to avoid in part and in part explain. 



I shall take up one by one the beds as seen in Minnesota and 

 compare them with the same in Wisconsin, so far as I can. 



The lower Trenton limestone, or Trenton limestone of Min- 

 nesota, consists of three beds differing somewhat in lithological 

 character and fauna; most strongly so in the area around the 

 'Twin Cities," /. e., Minneapolis and Saint Paul. The first of 

 these, next to and conformable with the Saint Peter sandstone, 

 is the same bed as the Lower Buff* limestone of Wisconsin, judg- 



