178 THE CAMBRIAN. [hull. 81. 



He then enters upon the discussion of the relative age of the sand- 

 stones of Wisconsin and those of Lake Superior, and concludes that 

 while the older beds of the latter area are apparently below the fossil- 

 iferous beds of the Upper Mississippi Valley, the newer sandstone of 

 the St. Mary's Eiver, which is apparently of the age of the St. Peter 

 sandstoue, or the Chazy formation, will be found overlying the fossil- 

 iferous sandstone, either with or without the intervention of the Lower 

 Magnesian limestone. 1 



On a geological map of Wisconsin published in 1855 Mr. I. A. Lapham 

 delineates the distribution of a sandstone occurring beneath the Lower 

 Magnesian limestone. Under the same color he includes the sandstone 

 on the south shore of Lake Superior, both on the north and southeastern 

 side of the trap rocks of Keweenaw Point. The distribution of the 

 fossiliferous sandstone south and southeast and east of the northern 

 area of the primitive rocks is also laid down. 2 In a new geological 

 map of Wisconsin, published in 1869, only the sandstones of the north- 

 western portion of the State are colored as Potsdam sandstone. In the 

 vicinity of the shores of Lake Superior the Michigan area on the map of 

 1855 is left un colored. 



A paper on.the age of the quartzites, schists, and conglomerates of 

 Sauk County, Wisconsin, by Prof. B. D. Irving, proves conclusively 

 that the quartzites are of pre-Potsdam age, and that the latter are de- 

 posited unconformably upon the flanks of the hill formed by the quartz- 

 ites. 3 The evidence given by Prof. Irving negatives the conclusion ar- 

 rived at by Dr. Alexander W r inehell in his description of tbe fossils of 

 Sauk County, where he states that Ptychaspis and Dikelocephalus 

 occur at the base of the Potsdam, and that the quartzites in question 

 are the lower beds of the Potsdam formation. 4 In the region of south- 

 central Wisconsin, especially in Dane and Columbia Counties, Prof. 

 Irving obtained a thickness of 800 feet for the lower or Potsdam sand- 

 stone, and for the superjacent Mendota limestone 30 feet, above which 

 occurs the Madison sandstone, with a thickness of 35 feet. He refers 

 only the lower or Potsdam sandstone to the Primordial. 5 



The sketch on the geology of northern Wisconsin by Mr. E. T. Sweet 

 gives a general description of the Potsdam sandstone, and attention is 

 called to the fact that all the strata at the falls of St. Croix belong to 

 this sandstone formation and that none of them are of igneous origin, 

 as suggested by Dr. Owen. 6 



»Op. cit.,p. 220. 



3 Lapham, L A.: Geological Map of Wisconsin. Milwaukee, 1855. Also a map for 1869. 



8 On the age of the quartzites, schists, and conglomerates of Sauk County, Wisconsin. Am. Jour. 

 Sci ., 3d ser., vol. 3, 1872, pp. 95-99. Wise. Acad. Sci. Trans., vol. 1, 1872, pp. 129-137. 



4 Notice of a small collection of fossils from the Potsdam sandstone of Wisconsin and the Lake 

 Superior sandstone of Michigan. Am. Jour. Sci., 2d ser., vol. 37, 1864, pp. 226-232. 



6 Note on some new points in the elementary stratification of the Primordial and Canadian rocks of 

 south-central Wisconsin. Am. Jour. Sci., 3d ser., vol. 9, 1875, pp. 441, 443. 



6 Notes on the geology of northern Wisconsin. Wise. Acad. Sci. Trans., vol. 3, 1876, p. 51. 



