184 SIXTEENTH REPOET ON THE CABINET OP NAT. HISTORY. 



Comparatively few species have been determined from authentic 

 Potsdam sandstone in its more eastern localities, although nu- 

 merous forms have been described from calcareous and shaly beds, 

 regarded by some geologists as equivalent, by others as newer, and 

 by still others as older than the Potsdam sandstone. I have made 

 no comparisons with these, since they appear to me all entirely 

 distinct from those which I have enumerated and described from 

 the sandstone of the Mississippi valley, and approached in cha- 

 racter only by the later forms of that period. 



In this connexion, moreover, our attention is directed to the 

 fact that this sandstone of the Mississippi valley, in its eastern 

 extension, has greatly thinned towards the outlet of Lake Superior; 

 and from observations on the Escanaba and Menomonee rivers, I 

 am satisfied that its thickness is greatly reduced, there being but 

 a narrow space between the Lower Silurian limestones and the 

 crystalline rocks. These crystalline rocks of the south shore of 

 Lake Superior had assumed essentially their present condition 

 before the deposition of the Potsdam sandstone ; the ocean on 

 the west of this barrier was, in a measure, separated from that 

 on the east ; and the distribution of the fauna has probably been 

 influenced by the same causes. 



Notwithstanding the successive stages recognized, the physical 

 conditions have been very monotonous throughout the entire 

 period ; and in the character of the fauna, there are similar in- 

 dications. We find great numbers of individuals of one species ; 

 and although recognizing very distinctly numerous species, there 

 is a kind of uniformity of character and monotony of expression, 

 never equalled by so many species in any formation of equal 

 thickness ; or even of much less thickness, where consisting of 

 varied character and conditions of deposit. 



The multitude of individuals of a few species is really wonder- 

 ful ; for in some beds the layers may be separated at every inch, 

 or even half-inch, and yet the entire surface is covered with the 

 dismembered parts of these ancient trilobites. 



In the preparation of this paper, I have received much valuable assistance 

 from Mr. K. P. WniTriELD, whose discrimination in distinguishing the 

 fragmentary species, and mechanical skill in disentangling these fragile 

 materials from a friable sandstone, as well as accurately representing them 

 in the illustrations, have enabled me to describe and characterize species 

 which otherwise would, at the present time, have been left untouched. 



