176 THE CAMBKIAN. Tbull.81. 



plain through Canada to the outlet of Lake Superior and along the south shore of 

 that lake. 1 



This is followed by a general account of the formation and of the or- 

 ganic remains found in it. 2 



In the same volume Prof. J. D. Whitney prints a brief resume* of the 

 Potsdam or lower sandstone. 3 



In a paper entitled "Preliminary notice of the fauna of the Potsdam 

 sandstone," Prof. James Hall states that in 1850 he had the opportunity 

 of tracing the formations from Drummond's Island and St. Mary's Kiver 

 to the head of Green Bay and thence across the country to the Missis- 

 sippi River. 4 He comes to this conclusion : 



The position of the sandstone on the St. Mary's admitted of no doubt, and its rela- 

 tive position to the lower limestone had before that time been well determined, and 

 the same was likewise ascertained by the several exploring parties along different 

 lines between Lake Superior and Green Bay. 



Throughout Wisconsin there is no difficulty in recognizing the following sequence: 



Trenton limestone ; 



Black River or Buff limestone; 



Birdseye limestone; 



St. Peter's sandstone; 



Lower Magnesian limestone, or calciferous sandstone; 



Potsdam sandstone. 



The St. Peter's sandstone holds the place of the Chazy limestone of the more 

 eastern localities, and, with this exception, we have the same sequence that we find 

 in New York, many of the fossils being common to the limestone of New York and 

 Wisconsin. 



Dr. Owen, in his published report, has adopted this view of the sequence, and the 

 explorations of subsequent years have confirmed the opinions then entertained ; and 

 I believe at this time every geologist will admit the ideutity of the Potsdam sand- 

 stone of New York and the Lower sandstone of the Upper Mississippi Valley. 



In speaking of this sandstone I shall, therefore, without hesitation, refer to it as 

 the Potsdam sandstone. 5 



A list of fossils described in the memoir is given to illustrate their 

 stratigraphic position. 6 In this list the fauna is divided into that of 

 the lower beds, middle beds, and upper beds, the latter characterized 

 especially by the presence of the genus Dikelocephalns. In a supple- 

 mentary note on the Potsdam sandstone that accompanies the memoir 

 he gives a resume of the evidence upon which the lower sandstone of 

 the Upper Mississippi Valley has been placed in parallelism with the 

 sandstone of New York, known as the Potsdam sandstone, as follows: 7 



In comparing the older rocks of New York and of the East generally with those of 

 the West, it should not be forgotten that there is a long interval on the line of the 



1 Physical geography and general geology. Geol. Surv. Wisconsin, Report, vol. 1, 1862, p. 14. 



2 Op.cit. , pp. 20-23. 



'Stratigraphical geology. Geol. Surv. Wisconsin, Rep. , vol. 1, 1862, pp. 140-144. 



♦Preliminary notice of the fauna of the Potsdam sandstone, with remarks on the previously known 

 species of foasils'and descriptions of some new ones from the sandstone of the Upper Mississippi Val- 

 ley. 16th Ann. Rep. Regents Univ. N. Y., State Cab. Nat. Hist , 1863, pp. 119, 120. 



6 Op. cit, p. 120. 



«Op.cit.,p.209. 



T Op. cit, pp. 211-218. 



