PHYSICAL AND FAUNAL EVOLUTION 79 



St. Clair' limestone of Arkansas. The Alexandrian series of Savage 

 contains many types unknown from the true Niagaran, some 

 Ordovicic genera also being present (Raiinesquina, Platystrophia, 

 Rhynchotreta, Zygospira). Few typical Niagaran species occur, 

 but the presence of the genera Favosites, Atrypa, Whitfieldella, 

 Homoeospira, Schuchertella, Chlorinda, and Lichas (Metopolichas) 

 indicates the Siluric age of this fauna. It probably represents an 

 invasion from the south before the Niagaran transgression from the 

 north had reached the southern Illinois region. Northward, in central 

 and northern Illinois, this fauna seems to be wanting, the true 

 Niagaran fauna here succeeding the Cincinnatian. 



The Alexandrian is succeeded disconformably by 30 to 75 feet 

 of limestones with a Lower Niagaran fauna. A transgression is 

 indicated by the fact that "where the formation is thinnest, it is the 

 lower, and not the upper layers that are absent."^ The Niagaran 

 fauna includes: Favosites favosus, Halysites cafemilatus, Atrypa 

 rugosa, Orthis jiahellites, O. cf. davidsoni, Plectambonites transversalis, 

 Stricklandinia triplesiana, and Triplesia ortoni; which grouping, as 

 stated by Savage, corresponds to that of the Clinton of the Dayton, 

 Ohio, region. 



The invasion of the interior by a southern fauna, in later Niagaran 

 time, seems to be indicated by the later Siluric formations of Tennessee 

 and possibly in part by the Louisville limestone of Indiana and 

 Kentucky. The higher beds of western Tennessee, called by 

 Foerste^ the Brownsport beds, and subdivided into the Beech River, 

 Bob, and Lobelville formations by Pate and Bassler"* contain faunas 

 apparently not found in the typical or northern Niagaran formations, 

 and W'hich are well developed in the underlying series, named, in 

 ascending order, Clinton, Oswego, Laurel, Waldron, Lego, and 

 Dixon. 



1 Gilbert Van Ingen, "The Siluric Fauna near Batesville, Ark., Part I," School 

 of Mines Quarterly, Vol. XXII (April, 1901), pp. 318-29. 



2 Savage, op. cit., p. 435. 



3 A. F. Foerste, "Silurian and Devonian Limestones of Western Tennessee," 

 Jour. Geol., Vol. XI, pp. 554-715. 



4 W. F. Pate and R. S. Bassler, "The Late Niagaran Strata of West Tennessee," 

 Proc. U. S. Nat. Mus., Vol. XXXIV, pp. 407-32. See also Roemer, Die silurische 

 Fauna des westlichen Tennessee, in which the fauna of these higher beds is described. 



