Hi THE PALEONTOLOGY OF MINNESOTA. 



much of the underlying shale should be thus included. When, however, it was found that 

 the Galena limestone proper had no uniform base line, but that its basal portion became 

 shale toward the north and that other shale beds began to be interbedded in the limestone 

 at higher levels, it was apparent that in Minnesota there was nothing left of a lithological 

 base line, and that the only criterion on which to establish the bottom of the Galena was a 

 downward extension of its characteristic fossils, and an enumeration of the other associated 

 and characteristic species. Thus it became apparent that in Minnesota about thirty feet on- 

 ly of the underlying shales might be put into the Galena formation. In Iowa there is reason 

 to believe that a greater thickness of the underlying strata may thus be transferred to the 

 Galena. Owing, however, to the gradual lithologic transition from shales to limestone, 

 or vice versa whether horizontally or perpendicularly it is apparent, as already remarked 

 that the characteristic fossils of the Galena and their associates will not be found to be 

 distributed throughout the region in complete conformity with the limits here established 

 and the query very naturally arises whether the distinction between the Galena and the 

 Trenton is one which on any terms, whether lithological or paleontological, ought to be 

 perpetuated. Our results certainly show so intimate a relation between them that they 

 might with propriety be put into the same formation with a common designation. 



It is barely necessary to call attention to other conclusions that spring from an in- 

 spection of this table and a comparison of it with the tabulation of fossil species given 

 in the introduction to part n of this volume. 



The suggestion of D. D. Owen in 1852 that the Galena can be parallelized with the 

 Utica slate and Hudson River formations finds no support in our results, but those forma- 

 tions are necessarily at a higher horizon if they both occur in the Northwest. 



The suggestion of H. D. Rogers, in 1858, that the Galena limestone is possibly the 

 western representative of the Utica slate, more elaborated and adopted by C. D. Walcott 

 in 1879, is not supported by our results. 



The terms Buff and Blue, used to designate some portions of these formations (Tren- 

 ton), under the erroneous idea that the strata to which they were applied were of the age 

 of the Blue limestone of Ohio (Cincinnati group), have been the source of many mistakes; 

 and as the strata are older than their supposed equivalents, these terms ought not to be 

 further employed. 



There are good reasons for believing that the Hudson River was separated from the 

 Galena, or top of the Trenton, in the Northwest, by some physical convulsion which ex- 

 terminated, or expelled, most of the species that preceded it. This is indicated not only 

 by the rarity of the species that survived the change but by certain physical features that 

 accompany the basal beds of the Hudson River. At Maquoketa, Iowa, Mr. James mentions 

 some evidence of stratigraphic non-conformity at this horizon. From this horizon upward 

 into the limestones of the Upper Silurian the transition is not more marked than from the 

 Galena to the Hudson River. 



