212 SIXTEENTH REPORT ON THE CABINET OF NAT. HISTORY. 



the St.Peters sandstone of "Wisconsin and Minnesota ; and it is 

 this sandstone, doubtless, which has been taken for the Potsdam 

 sandstone in some localities along that line. 



The succeeding Birdseye and Black-river formation, from La- 

 cloche to Lake Superior, has become a buff-colored magnesian 

 limestone, or weathering externally to this color, but still holding 

 the characteristic fossils. 



In New- York, a sandstone (the Potsdam) lies immediately be- 

 neath a magnesian limestone (the "Calciferous sandrock") : this 

 deposit is succeeded by a calcareous formation (the Chazy), in- 

 cluding a sandstone, and surmounted by the Birdseye, Black-river 

 and Trenton limestones. 



In Wisconsin, Iowa and Minnesota, we have undoubted Trenton 

 limestone, and below it a buff-colored magnesian limestone con- 

 taining so many of the characteristic fossils of the Birdseye and 

 Black-river limestones as to leave no doubt of the parallelism of 

 these beds with those of New-York. Below this magne^iian lime- 

 stone we have the St.Peters sandstone, corresponding, as already 

 shown, with the Chazy formation ; and beneath this a magnesian 

 limestone, which, in its position and lithological character, cor- 

 responds in all respects with the ''Calciferous sandrock-^ of New- 

 York. 



It is from all these facts, that the lower sandstone of the Upper 

 Mississippi valley has been placed in parallelism with the sand- 

 stone of New-York known as the "Potsdam." 



Notwithstanding however that this sequence is precisely like 

 that observed in New- York, it may not yet be regarded as proved 

 that the sandstone, from which I have described these fossils, is in 

 all respects the equivalent of the Potsdam sandstone of New-York, 

 Vermont and Canada. It may represent more, or it may represent 

 less, than that formation. The lower accessible beds of the Mis- 

 sissippi valley may represent the Potsdam of one hundred and 

 fifty or two hundred feet in thickness in the typical localities in 

 New- York, while the middle and upper beds of the West may be 

 of epochs not represented in that part of the series studied in 

 New- York; and in some other places, as in the regions just men- 

 tioned, the same epochs may be represented by a shaly or semi- 

 calcareous deposition, or may be included in the commencement 

 of the Calciferous epoch. It should not therefore be regarded as 

 decided that the Potsdam sandstone, as developed in New- York, 

 occupies the entire interval from the base of the oldest sedimen- 

 tary formation of the palasozoic era, to the Calciferous sandstone. 



