514 T. E. Savage — Ordovician and Silurian Formations. 



quadrata, Hebertella insculpta, Plectorthis whitfleldi, Stro- 

 phomena fluctuosa and S. planumbona. Only one of these 

 recurs in either of the higher horizons. From these consider- 

 ations the Fernvale formation of southern Illinois is thought 

 to correspond, in time, with the lower Rhynchotrema capax 

 horizon of the Maquoketa beds in Fayette county, Iowa, 



Outside of this region the Fernvale formation is known in 

 Illinois from Monroe county ; and it has also recently been 

 recognized by the writer in the vicinity of Millsdale, and again 

 two miles further north, in Will county. 



The Post- Fernvale Unconformity. 



The presence of an unconformity between the Fernvale 

 limestone and the overlying sandstone is shown in the fact 

 that at some points in adjacent portions of Missouri the Thebes 

 sandstone formation rests upon the weathered surface of the 

 Kimmswick beds, the Fernvale strata being entirely absent. 

 A considerable movement is also indicated in the change from 

 the limestone strata of the Fernvale to the sandstone of the 

 succeeding formation. 



The Thebes Sandstone. 



The name " Thebes sandstone" was given by Worthen to 

 the chocolate-colored sandstone and sandy shale which is well 

 developed and favorably exposed in the town of Thebes. The 

 formation is separated by its lithology and fauna, and also by 

 an erosional unconformity, from the Fernvale limestone upon 

 which it rests, and from the overlying calcareous shale. In 

 its lower part the strata consist of a few feet of fine, slightly 

 shaly sandstone, above which the beds become more massive 

 and the texture more coarse. In the upper portion the mate- 

 rial w T eathers into thin flakes or flaglike layers, and contains a 

 small admixture of shale. The thickness of the formation is 

 about 75 feet. This sandstone carries a meager fauna. In a 

 narrow zone near the base trilobite fragments are very abun- 

 dant, but throughout the greater portion of the thickness an 

 occasional shell of Lingula covingtonensis, and branches of 

 Climacograptus putillus are the only fossils that are encoun- 

 tered. 



The Thebes sandstone is exposed over a much larger area in 

 this region than any of the preceding formations. It has also 

 recently been found to have a much wider distribution in the 

 state than was formerly supposed. 



The Post-Thebes Unconformity. 



Evidence of a break in sedimentation closing the deposition 

 of the Thebes formation appears in the abrupt change in the 



