LOWER SILURIAN SHALES 39 



The upper Trenton, or Trenton proper, is well exposed in 

 all the counties along the Mississippi river from Marion to 

 Cape Girardeau. In its northern extension it is chiefly a buff- 

 colored or yellowish-gray limestone with occasional shale part- 

 ings. Fossils are abundant in places, though often in the form 

 of casts. Southward the limestone becomes compact, bluish- 

 drab, with abundant fossils. 



HUDSON SHALES. 



Everywhere on the eastern border of Missouri, wherever 

 the Trenton limestone is exposed, blue calcareous shales are 

 found to overlie it. These shales rapidly disintegrate, 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 Shu- 

 mard early correlated these shales with the Hudson River 

 shales of I^ew York and Ohio. In 1868 Worthen* called these 

 beds the " Thebes " shales, from the village of that name in 

 southern Illinois, on the Mississippi river below Cape Girar- 

 deau. 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 Shumardf, but placed by the 

 last author in the Upper Silurian. In the same region Shu- 

 mard | had previously ( though through delays not published 

 until several years later) divided those shales into: 



Upper Hudson shales 45 feet 



Cape Girardeau sandstone 35 feet 



Lower Hudson shales 50 feet 



At Thebes the whole formation is well exposed in a sharp 

 anticline, bringing up centrally the Trenton limestone above 



*Geol. Sur. lUinois, Vol. Ill, p. 27. 1868. 



t Geol. SuT. MiBsonri, 1 & 2 Ann. Kep., p. 154. 1855. 



JGeol. Sar. Missouri, Rep. 1855-1871, p. 26i. 1873. 



