Contributions to Paleontology. 



183 



roidal 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 lime- 

 stone of Wisconsin and Iowa, with a thickness of two 

 hundred and thirty feet. 1 Below this, therefore, if the 

 depositions were uniform, we should find the sandstone as 

 seen on the Upper Mississippi; but instead of a continuous 

 arenaceous formation, we have as follows : 



Second sandstone 70 feet.* 2 



Third Magnesian limestone... 350 " 



Third sandstone 50 " 



Fourth Magnesian limestone. 300 " 



We look in vain, therefore, for that great development 

 of arenaceous sediments at this period, which we find 

 farther to the north in the Mississippi valley. 3 



Considering this great augmentation of magnesian lime- 

 stone towards the south, and the largely increased thick- 

 ness of the sandstone farther north, we might be prepared 



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 limestone. 



!Dr. Shumard, however, considers the Third Magnesian limestone as the 

 equivalent of the "Calciferous sandstone" of New York and of the 

 "Lower Magnesian limestone " of Iowa and Wisconsin. This opinion is 

 founded on identical or similar forms of fossils ; and admitting this evi- 

 dence, it will necessarily modify the conclusions which have heretofore 

 been drawn from the interpreted sequences. 



2 Dr. Shumard has given this sandstone as attaining a thickness of one 

 hundred and twenty t o on e hundred and forty feet in some localities ( Geologi- 

 cal Report of Missouri, p. 166). 



3 If we take the First Magnesian limestone of Prof. Swallow as the 

 representative or equivalent of the Eastern Calciferous sandstone, it will 

 not materially alter the general conclusion ; for we add but 125 feet of 

 sandstone, with 190 feet of magnesian limestone, above the beds just 

 cited. 



