THE LOWER SILURIAN ROCKS. 527 



and the brown-colored Lower Magnesian area being occupied by the 

 Madison and Mendota beds. In the sections also, both of the Atlas 

 plates and of the plates in this volume, the Mendota and Madison 

 beds are given separately from the Potsdam. Having been mapped 

 out with some care, it was thought best not to lose the work done on 

 them by not so distinguishing them. Moreover, the close likeness 

 often borne by the Mendota to the Lower Magnesian, and the gradu- 

 ation of one series into the other, render it a matter of doubt whether 

 to affiliate these beds of passage with the upper or lower of the two 

 series. Constituting so important a feature as they do in the Central 

 Wisconsin stratigraphy, they deserve separate mention; they are 

 really beds of passage, and as such are separately considered below. 

 The term Potsdam, then, as used in the detailed descriptions and sec- 

 tions of this chapter, applies only to those beds terminating upwards 

 at the Mendota base. 



Of the two names given to the series the term " Lower Sand- 

 stone" was used by Dr. D. D. Owen, as distinguishing it from an- 

 other much thinner, but very prominent, sandstone, which overlies 

 the Lower Magnesian the Upper or St. Peters sandstone. Hall and 

 Whitney first used the term "Potsdam,'* transferring it from the 

 sandstone series which .forms the Silurian base in the state of New 

 York, and contains a few fossils close to those in the Wisconsin beds, 

 which, however, contain many that are not found in New York. 

 That the two formations are somewhere nearly the equivalents of one 

 another appears evident, as Hall has shown. 1 The extension of the 

 New York name, originally given to a comparatively small thickness 

 of rock which occupies a restricted area, to the lowest of the fossil- 

 iferous Silurian beds all over the country, seems, however, unfortunate, 

 and especially so in the case of the Wisconsin formation, which has 

 a thickness of 800 to 1,000 feet, comes to the surface over an extended 

 area, and is far more fossiliferous than the New York beds. 



Of former investigations on the Potsdam series, Dr. Owen's seem 

 to have been much the most exhaustive. He presents a scheme of 

 the subordinate structure of the formation which may be considered 

 quite remarkable for so early a day. His detailed investigations, 

 however, did not extend far away from each side of the Mississippi, 

 and the great central area of Wisconsin, where the formation spreads 

 over a district 100 miles in diameter, and presents elements of strat- 

 ification contrasting much with those exhibited along the Mississippi, 

 he left hardly touched. Having no data from Artesian borings, he 

 greatly underestimated the thickness of the formation, putting it at 



1 16th Annual Report N. Y. State Cabinet Natural History. 



