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 characteristicspecies. Thus it became apparent that in Minnesota about thirty feet on- 
ly of the underlying shalesmight be put into the Galenaformation. 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 11 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, lowa, 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. 
