170 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



base) and Kimmswick limestone, and Keyes's Hudson is now called Thebes forma- 

 tion. The formation overlying the Thebes is known as Girardeau limestone. 

 According to Uh-ich " the Thebes formation includes more than the Maquoketa 

 shale, recognized farther north in Missouri and elsewhere. 



Keyes described the three higher Ordovician formations as follows: 



Trenton limestone. — In southeastern Missouri the Trenton limestone as now understood 

 embraces, besides the Trenton as comprehended in earlier reports of the State, the Black River 

 and Birdseye limestones of Shumard. The latter probably more properly represents the lower 

 and less fossiliferous portion of the Trenton of the region and nowhere can be separated 

 faunally or lithologically from the upper part containmg the typical Trenton fauna. 



The lower Trenton ("Black River" and "Birdseye" limestone) is a compact, heavily 

 bedded limerock, often not unlike certain lithographic stones in texture. 



The upper Trenton, or Trenton proper, is well exposed in all the counties along the Mis- 

 sissippi River from Marion to Cape Girardeau. In its northern ex;tension it is chiefly a buff- 

 colored or yellowish-gray limestone with occasional shale partings. Fossils are abundant 

 in places, though often in the form of casts. Southward the limestone becomes compact, 

 bluish-drab, with abundant fossils. 



Hudson sTwbles. — Everywhere on the eastern border of Missouri, wherever the Trenton 

 limestone is exposed, blue calcareous shales are found to overlie it. These shales rapidly dis- 

 integrate, upon exposure to the weather, into a soft plastic clay. Numerous thin seams of 

 impure limestone are intercalated and often form beds of considerable thickness. Fossils are 

 abundant and well preserved. They are all very characteristic of the fauna occurring at 

 Cincinnati, Richmond (Indiana), and in northeastern Iowa. 



Upon lithological and faunal grounds. Swallow and Shumard early correlated these shales 

 with the Hudson River shales of New York and Ohio. In 1868 Worthen called these beds 

 the "Thebes" shales, from the village of that name in southern IlUnois, on the Mississippi River 

 below Cape Girardeau. As defined by the Illinois geologist, the Thebes shales and sandstone 

 form the lowest member of the Cincinnati group, the upper section embracing practically the 

 same beds as the Girardeau limestone of Shumard but placed by the last author in the Upper 

 Silurian. In the same region Shumard had previously (though through delays not published 

 until several years later) divided those shales into — 



Feet. 



Upper Hudson shales 45 



Cape Girardeau sandstone 35 



Lower Hudson shales 50 



At Thebes the whole formation is well exposed in a sharp anticline, bringing up centrally 

 the Trenton limestone above the water level of the Mississippi River, and successively, on 

 either side, all the beds of the Hudson shales. 



North of the Missouri River, in Pike, Ralls, and Marion counties, the lithological characters 

 and fossils are essentially the same as in the southern part of the State. 



Girardeau limestone. — This rather well marked division of the lower Silurian in south- 

 eastern Missouri was first differentiated by Shumard in 1885 and provisionally called by him 

 the Cape Girardeau limestone but was regarded as a member of the Upper instead of the Lower 

 Silurian. 



Lithologically the limestone is bluish, very compact, and resembles somewhat the stones 

 used in lithographing. It is rather thinly bedded, with numerous vertical fractures or joints. 

 Fossils of peculiar types abound. Its thickness is over 60 feet. 



Worthen also recognized this formation as a distinct horizon but made it the upper 

 member of the Cincinnati group, the superior part of the Lower Silurian of the region. 



- Personal communication. 



