THE 



AMERICAN JOURNALOFSCIENCE 



[FOURTH SERIES.] 



Art. XLYII. — The Ordovician and Silurian Formations in 

 Alexander County, Illinois ;* by T. E. Savage. 



location and Earlier Work. 



Rocks of Ordovician and Silurian age are exposed in south- 

 west Illinois only over a narrow belt, less than four miles in 

 maximum width, bordering the Mississippi river. The line of 

 outcrop of these strata extends along the west side of Alex- 

 ander county, and continues north about two miles into the 

 southwest corner of Union. 



For almost forty years practically no work was done on 

 these horizons in this portion of the state. In 1866, Worthenf 

 described a bed of massive, light gray, semi-crystalline lime- 

 stone, outcropping near Thebes, as the lowest strata exposed 

 in this part of Illinois, and correctly referred it to the Trenton 

 (=Mohawkian) series. Concerning the Cincinnatjan strata in 

 this region he says :% 



" They consist of about 100 feet in thickness of brown, 

 sandy shales and regularly bedded, brown sandstone (Thebes 

 sandstone and shale) which forms the lower portion of the 

 group ; overlain by about forty feet of thin-bedded, com- 

 pact, fine-grained limestone — which breaks with smooth, con- 

 choidal fracture " (Cape Girardeau limestone). 



Under the name Clear Creek limestone§ he described a group 

 of siliceous limestones in this region which immediately suc- 

 ceed the G-irardeau. These he interpreted as occupying the 

 same stratigraphic position as the Niagara dolomites in the 

 northern part of the state. 



* Published by permission of the Director of the Illinois Geological Survey. 



f Worthen : Geol. Surv. 111., vol. i, p. 148. 



% Ibid., p. 139. § Ibid., p. 126. 



Am. Jour. Sci.— Fourth Series, Vol. XXVIII, No. 168.— December, 1909. 

 34 



