Contributions to Palceontology. 



181 



doubt of the parallelism of these beds with those of New 

 York. Below this magnesian limestone we have the St. 

 Peters sandstone, corresponding, as already shown, with 

 the Chazy formation; and beneath this a magnesian lime- 

 stone, which, in its position and lithological character, 

 corresponds 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 sandstone of New York known as the "Pots- 

 dam." 



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 forma- 

 tion. The lower accessible beds of the Mississippi 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 

 maybe of epochs not represented in that part of the series 

 studied in New York; and in some other places, as in the 

 regions just mentioned, the |same epochs may be repre- 

 sented 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 sedimentary 

 formation of the palsezoic era, to the Calciferous sandstone. 

 From what we know of the -primordial fauna in other 

 localities, 1 we are prepared to find beds above or below, or 



1 This formation, in Canada, has a thickness of between six and seven hun- 

 dred feet [Geology of Canada, pp. 88 & 89); but even there it is not sup- 

 posed to represent the entire primordial zone. Nor does the fauna, at 



