144 IOWA ACADEMY OF SCIENCE 



be a local modification as noted by Chamberlin. 3 The Sparta 

 beds differ from the Mendota limestone in several respects, as 

 follows : 



1. The maximum thickness of the Mendota at the type lo- 

 cality at Madison is thirty-five feet 4 while the maximum known 

 thickness is eighty feet. 5 The Sparta beds reach a maximum 

 thickness of two hundred feet. 



2. The Mendota member has been recognized as a limestone 

 or a calcareous horizon 6 in the upper part of the Potsdam, and, 

 as such, should effervesce upon the application of acid. No 

 such action takes place when the acid is applied to the Sparta 

 beds, although there are minor concentrations of calcium car- 

 bonate which would undoubtedly respond to acid. An analy- 

 sis of the Mendota limestone given by Irving 7 bears a close sim- 

 ilarity to the Prairie du Chien (Lower Magnesian) formation. 

 This analysis shows a high percentage of lime carbonate. As 

 stated above, the Sparta member contains more than fifty per 

 cent sand. 



3. The Mendota beds have been considered as a horizon for 

 trilobites, 8 and where typically exposed numerous trilobite re- 

 mains have been found. Although many exposures of the Spar- 

 ta beds were examined carefully, no trace of trilobite remains 

 was found. The presence of certain species of brachiopods be- 

 longing to the Lingula group 9 also characterizes the Mendota 

 horizon. The fauna of the Sparta beds includes an uncertain 

 species of Obolella and the impressions of a doubtful Orthis. 

 Faunally, there is a difference between the Mendota and the 

 Sparta beds. 



The Sparta beds do not seem to be the equivalent of the Saint 

 Lawrence member since Calvin 10 recognized the Saint Lawrence 

 member as evenly bedded calcareous strata corresponding to the 

 fifth trilobite bed of Owen and attaining a thickness of thirty- 

 five feet. 



In a recent publication of the Wisconsin Geological and Nat- 

 ural History Survey, 11 the Upper Cambrian is subdivided into 



3 Geology of Wisconsin, Vol. I, pp. 121-122. 



*Bull. XIII, Wis. Geol. and Nat. Hist. Surv., p. 92. 



5 Geology of Wisconsin, Vol. II, p. 259. 



"Bull. VIII, Wis. Geol. and Nat. Hist. Surv., p. 37. 



7 Geology of Wisconsin, Vol. II, pp. 543-544. 



s Geology of Wisconsin, Vol. IT, p. 261. 



°Geology of Wisconsin, Vol. II. i>. 261. 



10 Iowa Geol. Survey, Vol. IV, p. 59. 



"Bull. XXXV, Wisconsin Geol. and Nat. Hist. Survey, pp. 30-31. 



