PHYSICAL AND FAUNAL EVOLUTION 51 



is as sharply marked as that of aeoUan quartz sands found upon the 

 clear-swept limestone floors of some modern deserts.' In some cases, 

 however, there appears to be absolute conformity between the St. 

 Peter sandstone and the underlying dolomites, pointing to con- 

 tinuous deposition. Both in Wisconsin and in Minnesota, the lower 

 Magnesian beds are often slightly folded, and the lower St. Peter 

 sandstone is likewise involved in these folds^ (Fig. 4). The upper 

 St. Peter, however, and the overlying Stones River, which are per- 

 fectly conformable, are not involved in these folds. In Minne- 

 sota, the Oneota, New Richmond, and Shakopee formations have 

 a combined thickness of 105 to 260 feet. If the Jordan and St. 

 Lawrence beds are regarded 



^^^^^^^Ss^R. 



as Ordovicic, though they still 



contain Dicellocephalus, the 



thickness is increased to 190 ni^?//^^\v|V^:^^:^^viriy^l<^^ 



feet minimum or 673 feet 



maximum. The faunas of all 

 the beds of the Lower Mag- 

 nesian series indicate lowest 



Ordovicic and close relation- ^'^- 4--Showing the relationship of the 



/^ u • Upper Stones River (S. R.) and lower Beek- 



Ship to the Upper Cambric. mantown (B.) Beds of Minnesota and the 



In the Black River region, included St. Peter (St. P.) (Redrawn from 



Gushing records 20 to 60 Hall and Sardeson.) 



feet of lowest Beekmantown (Theresa), succeeded disconform- 

 ably by Upper Chazy (Pamelia and Lowville limestones). The 

 base is probably not exposed in this section, the basal sandstone, 

 called Potsdam by Gushing, being most likely of later age. In the 

 Mohawk Valley, 350 feet of Beekmantown (Little Falls dolomite) 

 is followed disconformably by Upper Ghazy (Lowville) ; but here, 

 too, the base of the Beekmantown is not shown, and hence the true 

 thickness is unknown. In the Lake Ghamplain region the Beekman- 

 town is 1,800 feet thick; in southern Pennsylvania 2,250 to 2,300 feet; 

 in central Pennsylvania nearly 2,500 feet ; and in the Arbuckle Moun- 

 tains of Oklahoma 1,250 feet. In all these localities, except central 

 Pennsylvania, the upper limit of the Beekmantown is marked by a dis- 



I Compare Zittel, Beitrdge zur Geologic und Palaeontologie der lybischen Wiiste. 

 ' Hall and Sardeson, Bull. Geol. Soc. Amer., Vol. III., pp. 354, 355. 



