walcott. J CORRELATION. 409 



mation and equivalent to 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 Hue. 



The succeeding Birdseye and Black River formation from Lacloche to Lake Supe- 

 rior has become a buff-colored magnesias limestone, or weathering externally to this 

 color, but still holding the characteristic fossils. 



In New York a sandstone (the Potsdam) lies immediately beneath a maguesian 

 limestone (the "Calciferous sandrock"); this deposit is succeeded by a calcareous 

 formation (the Chazy), including a sandstone ami 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 containing so many of the characteris- 

 tic 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 magnesian limestone 

 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 lith- 

 ological 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 sand- 

 stone of New York, Vermont, and Canada. It may represent more or it may repre- 

 sent less than that formation. The lower accessible beds of the Mississippi Valley 

 may represent the Potsdam of 150 or 200 feet in thickness in the typical localities in 

 New York, while the middle and upper beds of the West may be of epochs not repre- 

 sented 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 represented by a shaly or 

 semi-ealeareons deposition, or may be included in the commencement of the Calcifer- 

 ous epoch. It should not therefore be regarded as decided that the Potsdam sand- 

 stone, as developed in New York, occupies the entire interval from the base of the 

 oldest sedimentary formation of the Paleozoic era to the Calciferous sandstone. From 

 what we know of the Primordial fauna iu 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 Potsdam sandstone of New York, and which may still be of the same 

 period. 



Winchell. — It is stated by Prof. N. H. Winchell that — 



It has been abundantly proved that the red sandstones of Lake Superior, however 

 disturbed and changed locally, or however much increased in thickness by the agency 

 of volcanic outbursts, are the exact equivalents of the New York Potsdam. They 

 occupy the first position over the metamorphic slates of the Huronian rocks on which 

 they lie unconformably, and from which they differ in being but slightly and only 

 locally metamorphosed. They retain usually their evidently sedimentary characters, 

 and have not well preserved fossil remains. 1 



He also publishes a table showing the number of species common to 

 various Silurian formations of the Lake Superior district and New York 

 State, and calls attention to the fact that three species only are found 

 in the Lake Superior sandstone and the typical Potsdam sandstone of 

 New York. 



'General sketch of the geology of Minnesota. Geol. and Nat. Hist. Survey, Minnesota, 1st Ann, 

 Eep. for 1872, 1873, p. 68. 



