332 HALL AND SARDESON PALEOZOIC FORMATIONS OF MINNESOTA. 



Page 



The Slictopovella Bed : : !'ii 



The St'irfiijmra (or upper Blue) Bed 362 



The Fucoid Bed 363 



The Zygospira Bed 363 



The Orthisina Bed 364 



The Camarella Bed 364 



The Lingulasma Bed •i , '»"> 



The Madurea Bed .' 365 



The ( iincinnati Limestone and Shales 365 



The Maquoketa Beds 365 



Localities 365 



Structural Characters 365 



Lithologic Characters 366 



Paleontologic ( lharacters 366 



The Wykoff Beds 366 



Localities 366 



Structural Characters 366 



Paleontologic Characters 366 



The Devonian 367 



Localities 367 



Structural Characters 367 



Lithologic Characters .'!'J7 



Paleontology ; »< »~ 



Summary of the Stratigraphy 368 



Introduction. 



The rocks described in the following pages occupy the entire area of 

 southeastern Minnesota, some 13, 200 square miles in extent. They 

 stretch eastward from a straight line between Mankato and Hinckley to 

 the state of Wisconsin, and from Chengwatona southward to Iowa. 



The periods of geologic time represented by these formations are three, 

 viz. Cambrian, Silurian and Devonian. That portion of the Cambrian 

 exhibited is the upper, of the Silurian the lower, and of the Devonian 

 so thin a layer is present and so few fossils occur in it that we cannot 

 assign the rocks to any division of that group, but suppose them to lie- 

 long near the middle. 



These Paleozoic rocks are underlain by the Archean and Algonkian, 

 and lie beneath patches of Cretaceous and a covering of Quaternary 

 debris save in that extreme southeastern corner included within the 

 " driftless area " of Chamberlin * 



*See map and description in "The Driftless Area," et ■. by T. ('. Chamberlin and H. D. Salisbury, 

 Bth Ann. Rep. U. S. Geol. Surrey, 1885, pp. 205-322. 



