

WALCOTT.l WISCONSIN. 179 



A note on the age of the Crystalline rocks of Wisconsin by Prof. Irving 

 states that the exact junction between the Potsdam and the Huronian 

 formations is often to be seen, the almost loose sand of the Potsdam, 

 with numerous fossils, together with fragments derived from the 

 older rocks, lying upon and wedged in between the tilted edges of 

 Huronian quartzite, schist, or other rock. Exactly similar unconform- 

 abjlity is to be seen at the Dalles of the St. Groix River between the 

 Potsdam and copper-bearing series; the horizontal beds of the former, 

 filled with shells of Lingulella, lying directly upon the columar, inela- 

 phyre(?) of the latter series, the exact junction of the two series being 

 exposed. 1 



The description of the geology of eastern Wisconsin by Prof. T. 0. 



Chamberlin contains a detailed account of the " Potsdam sandstone." 



He says : 



That it is the exact equivalent of the Potsdam sandstone of New York, as would 

 seem to be implied by the name, is nob absolutely certain, but as the term has been 

 nsed to designate this formation in previous reports upon the geology of the State, 

 and as the weight of evidence and authority favors this view, the name Potsdam 

 sandstone will be used without further qualification in this report. 2 



The sandstone varies greatly in thickness, ranging from zero to about 

 1,000 feet. A detailed account of the formation is given with a list of 

 the fossils found in the sandstone. The Madison sandstone and Men- 

 dota limestone are included as subdivisions of the terrane. 



Reference to the description of the Lower or u Potsdam n sandstone 

 series of central Wisconsin by Prof. R. D. Irving, 3 will be made under 

 the description of the Potsdam terrane in Wisconsin. 



A general description of the mode of occurrence and character of the 

 Potsdam sandstone in the lead regions of northwestern Wisconsin was 

 published by Mr. Moses Strong in 1877. 4 



Upon the basis of work in St. Groix, Dunn, and adjacent counties, 

 Mr. L. C. Wooster subdivided the "Potsdam" as follows: 5 



The numbers indicate the distances below the Lower Magnesian limestone. 



(a) Upper calcareous band : This varies greatly in thickness, and is the probablo 

 northwestern equivalent of the Mendota limestone near Madison; 75 to 85 feet. 



(6) Lower calcareous band: The limestone characters and the thickness are more 

 uniformly persistent than in a; 14a to 195 feet. 



(c) Hudson trilobite beds : Quite rich in trilobites and brachiopods, including one 

 new species of the former, with several undetermined ones ; 150 to 200 feet. 



(d) Glauconite layers: These comprise those layers which are very rich in glauco- 

 nite. Crinoid stems were found in these at Hudson; 160 to 210 feet. 



Iu b, c, and d, the lesser distance from the Lower Magnesian is true for western St. 

 Croix County, while the greater is nearer true for points east. 



'Note on the age of the Crystalline rocks of Wisconsin. Am. Jour. Sci., 3d ser., vol. 13, 1877, p. 

 308. 



2 (Geology of eastern Wisconsin.) Geol. Wise, Surv. of 1873-77, vol. 2, 1877, p. 257. 



3 (Geology of central Wisconsin.) Geol. Wise, Surv. of 1873-77, vol. 2, 1877, pp. 525-607. 



4 Geol. Wise. Surv. of, 1873-'77, vol. 2, 1877, pp. 668-688. 



5 Work in St. Croix, Dunn, and adjacent counties. Geol. Surv. Wiscouain, Ann. Rep. for 1877, 1878 

 pp. 36, 37. 



