CONTRIBUTIONS TO PALEONTOLOGY. 



213 



From what we know of the primordial fauna in other localities, 

 we are prepared to find beds above or below, or both above and 

 below, the epoch represented (so far as now known) by the Pots- 

 dam sandstone of New- York, and which may still be of the same 

 period. ( See jYote at the foot of page 220.) 



The evidence from the undisturbed region of the Mississippi 

 valley shows that the period of the Potsdam sandstone, or a very 

 considerable part of it, inay be represented by a small proportion 

 of arenaceous matter. The section of the rocks of Missouri, given 

 by Prof. Swallow, shows four separate magnesian limestones* 

 and three distinct sandstones, below beds which he recognizes as 

 the Birdseye and Black-river limestones. Taking the upper of these 

 magnesian limestones to represent the base of the Birdseye and 

 Black-river limestonesf , and the upper " Saccharoidal sandstone'' 

 to represent the Chazy or St. Peters sandstone of the Upper 

 Mississippi valley, we would then have in the Second Magnesian 

 limestone a representative of the Calciferous sandstone, or Lower 

 Magnesian limestone of Wisconsin and Iowa, with a thickness of 

 two hundred and thirty feetj. Below this, therefore, if the de- 

 positions were uniform, we should find the sandstone as seen on 

 the Upper Mississippi ; but instead of a continuous arenaceous 

 formation, we have as follows : 



* The section given by Professor Swallow is as follows : 

 BLACK-RIVER AND BIRDSEYE LIMESTONE. 



First Magnesian limestone 190 feet. 



Saccharoidal sandstone 125 " 



Second Magnesian limestone 230 « 



Second sandstone 70 " 



Third Magnesian limestone 350 " 



Third sandstone 50 «» 



Fourth Magnesian limestone 300 «* 



o 





t Dr. Shumard has remarked (Geological Report of Missouri, Part ii, p. 160), that 

 the fossils of this higher Magnesian limestone *' are most like those of the Black-river 

 and Trenton groups." There can be no doubt but here, as well as elsewhere in the 

 West, this fossiliferous magnesian limestone is of the age of the Birdseye and Black- 

 river limestones. 



X Dr. Shumakd, however, considers the Third Magnesian limestone as the equivalent 

 of the •' Calciferous sandstone" of New-York and of the '• Lower Magnesian lime- 

 stone" of Iowa and "Wisconsin. This opinion is founded on identical or similar forms 

 of fossils; and admitting this evidence, it will necessarily modify the conclusions 

 which have heretofore been drawn from the interpreted sequence. 



