244 AMADEUS W. GRABAU 
St. Clair™ limestone of Arkansas. The Alexandrian series of Savage 
contains many types unknown from the true Niagaran, some 
Ordovicic genera also being present (Rafinesquina, 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 catenulatus, Atrypa 
rugosa, Orthis flabellites, O. cj. 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 which are well developed in the underlying series, named, in 
ascending order, Clinton, Oswego, Laurel, Waldron, Lego, and 
Dixon. 
t Van Ingen, Gilbert, ‘‘The Siluric Fauna near Batesville, Ark., Part I,’ Schoo 
of Mines Quarterly, Vol. XXII (April, 1901), pp. 318-29. 
a Savage, op. cit., p. 435. 
3 Foerste, A. F., “Silurian and Devonian Limestones of Western Tennessee,”’’ 
Jour. Geol., Vol. XI, pp. 554-715. 
4 Pate, W. F., and Bassler, R.S., ‘‘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. 
