314 



GWLOGY. 



In other words, it does not follow that because the conditions for lime- 

 stone formation gave place to the conditions for shale formation in 

 New York at a particular time, similar change's took place in the interior, 

 or in any other distant area, at the same time. 



When the above names were applied to the Ordovician formations 

 of Wisconsin, the third and fifth formations were so named on the 

 supposition that they were the approximate equivalents of the New 

 York formations of the same name, while the Galena limestone was 

 regarded as a local development of the upper member of the Trenton 

 group, in which were also included the ill-defined equivalents of the Bird's 



Fig. 133. — Starved reck, an isolated mass of St. Peters sandstone on the Illinois 

 river near La Salle. The texture of the rock is essentially the same as at the 

 present border of the formation, 125 miles or so farther north. 



Eye (Lowville) and Black River limestones of New. York. 1 Recently, 

 question has been raised relative to the correctness of this correlation. 2 

 It has been urged that the formation' named . Trenton in the upper 

 Mississippi basin corresponds to the Lowville and Black River beds 

 of New York, while the Galena is the exact equivalent of the Trenton 

 of New York. The validity of this correlation remains to be deter- 



1 See Geol. of la., Hall and Whitney, Vol. I, pp. 54-70, and Geol. of Wis., Vol. L 

 1862, pp. 31-55. 



2 See Ulrich and Winchell, Geol. and Nat. Hist. Surv. of Minn., Vol. Ill, Pt. II, 

 pp. 83-128. 



