

WALCOTT.) WISCONSIN. - 177 



northern outcrop of these ancient strata, between the St. Lawrence and the western 

 limit of Michigan on the Menomonee River, where we can expect little aid from 

 paleontology. The fossiliferous beds of these ancient formations in Wisconsin lie to 

 the west of what appears to have been a great promontory at the time of their 

 deposition, stretching southward from the region of Lake Superior far into the 

 ancient sea. The disconnection caused by this promontory between the East and 

 the West would of itself prepare us to expect a fauna differing in a great degree 

 from beds of corresponding age on the opposite sides. 



It has been shown, by the investigations of the Canadian Survey, that not only the 

 Potsdam sandstone, but all the fossiliferous beds below the Birdseye and Black River 

 limestones are absent from Kingston on Lake Ontario to Lacloche on Lake Huron. 

 From Lacloche to Lake Superior there is a sandstone coming in below the Birdseye 

 limestone, which, from its position, may be considered of the age of the Chazy 1 for- 

 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 line. 



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

 perior, 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 beneath a magnesian 

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

 formation (the Chazy), including 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 containing so many of the characteristic 

 fossils of the Birdseye and Black River limestones as to leave no doubt of the paral- 

 lelism 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 for- 

 mation; and beneath this a magnesian limestone, which, in its position and litholog- 

 ical 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 formation. The lower accessible beds of the Mississippi Valley may repre- 

 sent 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 represented in 

 that part of the series studied in New York ; and in some other places, as in the re- 

 gions just mentioned, the same epochs may be represented by a shaly or semicalca- 

 reous 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 devel- 

 oped in New York, occupies the entire interval from the base of the oldest sedimen- 

 tary formation of the Paleozic era to the Calciferous sandstone. 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 Potsdam sandstone of New York, and which may still be of the same period. 



•The "Chazy formation" of the Canadian Geological Survey, in its eastern localities, includes a 

 sandstone which comes in below the greater part of the limestone, leaving from 10 to 20 feet of 

 shale a»d limestone beneath (Geology of Canada, 1863, p. 123*. It is apparently this sandstone of the 

 Chazy formation, having in Canada a thickness of 50 feet, which has become augmented in its west- 

 ern extension, while the calcareous part of the formation has partially or entirely disappeared. 



Bull. 81 12 



