PAUCITY OF LIFE IN THE MAGNESIAN. 



:;i!) 



be worked out. Crinoids, brachiopods, gasteropods, cephalopods, lamel- 

 libranchs, crustaceans, etc, have been found. In the Shakopee, numer- 

 ous specimens of Cryptozoon minnesotense, N. H. Winchell, occur both in 

 Minnesota and Wisconsin, but it is doubtful whether these should be 

 included as fossils on account of the difficulty of showing their organic 

 origin and of distinguishing them as they occur from merely folded strata 

 between which and the concretion-like Cryptozoon there seems to be every 

 degree of gradation. 



The Lower Silurian. 

 classification of the croc p. 



In its area and in the thickness and massiveness of its rocks this group 

 is greatly subordinate to the upper Cambrian in Minnesota; yet in 

 paleontologic interest it stands preeminent. Structurally and lithologic- 

 ally it is divided into limestones and shales. While these subdivisions 

 are sufficient for ordinary economic purposes, they are of no scientific 

 value; nor can they be, since they not only merge into one another but 

 both the limestones and the shales are very far removed from any type 

 both in physical character and chemical composition. A collection of 

 fossils such as lies before us, collected and arranged with much care, 

 develops the following classification of the formations : 



Lower Silurian < 



Cincinnati 



Trenton 



Galena. 



Trenton 



) Wykoff. ( Not subdivided.) 



I Maquoketa. (Not subdivided.) 

 Maclun a. 

 Lingulasma. 

 < 'amarelia. 

 Orthisina. 

 Zygospira. 

 Fucoid. 

 Stictopora. 

 Stictoporella. 

 j Blue limestone. 

 I Buff limestone. 

 Saint Peter, i Not subdivided.) 



In Minnesota t be Saint Peter consists of sandstones ; the Trenton and 

 < iincinnati of limestones and shales. So far as known to the writers, Mr. 

 E. < ). Qlrich was the first to apply the name "Trenton shales" to the 

 extensive series of calcareous shales occupying the upper part, from 

 Stictoporella to Zygospira, inclusive, of the division Trenton of the third 

 column above;. 



