THE PALEONTOLOGY OF MINNESOTA. 



most cases not distinguished, the .species of the Bluff limestone and of the Vanuxemia 

 bed being as a rule referred to the Trenton limestone or, more exactly, to the "lower 

 limestone of the Trenton formation." As we have said in discussing these beds on page 

 xcii, their faunas are not greatly different. Still as the fossils from each are readily 

 recognized in Minnesota by their modes of preservation, those of the lower bed retain 

 their shells while those of the Vanuxemia bed as a rule are casts merely we have care- 

 fully separated the species which, so far, appear to be characteristic of each. 



The greenish shales lying between the Vanuxemia bed and the yellowish or grayish 

 shales of the Clitambonites bed were divided into three unequal parts or thirds, "lower, 

 middle and upper thirds of the Trenton shales," corresponding in a general way with the 

 Stictoporella, Rhinidictya and Phylloporina beds. The Ctenodonta bed is occasionally 

 referred to as the "upper part of the middle third of the Trenton shales," while thePucoid 

 bed is sometimes called the "Orthis pectinella horizon." 



The " Prasopora insularis horizon " is the same as the Clitambonites bed, and it is this 

 bed that is usually meant when the text refers a fossil to the "Galena shales," though 

 that term frequently also includes more or less of the Fusispira bed. As a rule, however, 

 the shales of the latter bed are distinguished as the " upper part of the Galena shales. " 

 On the other hand, limestone deposits of the Fusispira bed in Fillmore county, the equiva- 

 lents, of which in Goodhue county are referred to as Galena shales, are included with the 

 rest of the Fusispira bed in the term "middle Galena.". In accordance with the sense of 

 the last term, the Galena or Trenton group was divided into three lithologic divisions, the 

 Galena shales, the middle Galena, a portion consisting principally of pure limestone, and 

 the upper Galena or Maclurea bed, a magnesian limestone. 



The Maclurea bed alone maintains the typical dolomitic character of the Galena, but 

 it diminishes in thickness from south to north, and may not have extended beyond 

 Goodhue county. The lithologic changes in the strata have caused a slight overlapping 

 in the designations of the special horizons. Thus the "Middle Galena," in speaking of 

 localities in Goodhue county, refers to the solid upper part of the Fusispira bed, while it 

 refers to the whole of the bed when Fillmore county localities are mentioned. 



It is to be remembered that the Galena shale is merely a lithologic phase moving 

 northward from county to county, and that it does not represent accurately any time 

 interval in the Trenton at large. If studied only in the region between Cannon Falls and 

 Berne the upper part of the shales would probably be separated as a distinct bed, as 

 indeed was done by Sardeson who called it " Camarella bed." But as this merges 

 gradually into the rest of the Fusispira bed, both lithologically and faunally, there is very 

 little reason, if any, for the sub-division. 



Occasionally reference is made in the volume to the Anastrophia bed, and the Upper 

 Clitambonites horizon. These refer to shales in Goodhue county immediately over the 

 Nematopora bed. It is the same horizon as the Platystrophia beds in Fillmore county, as 

 that term is used in the 19th annual report. 



