Contributions to Palceontology. 



177 



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 indications. We find great numbers of indi- 

 viduals 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 

 wonderful ; 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. R. P. Whitfield, whose discrimination in dis- 

 tinguishing the fragmentary species, and mechanical skill in disen- 

 tangling 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. 



[Trans. v.~] 23 



