354 T. E. SAVAGE — ALEXANDRIAN SERIES IN MISSOURI AND ILLINOIS 



On a later page of this report Worthen correlated with the Niagara of 

 northern Illinois the gray limestone forming the lower part of the Mis- 

 sissippi River bluff in the southwest part of Pike and the northwest part 

 of Calhoun counties in Illinois (now known to be of Sexton Creek or 

 Brassfield age). 



On another page he assigned to the Lower Helderberg group the dark 

 limestone and the overlying mottled red and gray limestone in the vi- 

 cinity of Thebes that represent respectively the Edgewood and the Sexton 

 Creek limestone in that portion of the State. He says : 



"It would appear probable that no beds of undoubted Niagara age were ever 

 laid down in southern Illinois, but in their places these siliceous limestones, 

 representing in part the age of the Lower Helderberg limestones and in part 

 the Oriskany sandstones of the New York series, were deposited resting 

 directly upon the Cincinnati group of the Lower Silurian." 



In a report on the Geology. of Alexander County, 8 Illinois, published in 

 1868, the Girardeau limestone is again assigned to the Cincinnati group, 

 while the overlying gray limestone, containing Dalmanites danai and its 

 associates, is referred to the Lower Helderberg. 



In a paper presented to the American Association for the Advancement 

 of Science in 1870 Worthen 9 again discussed the limestones occurring 

 between the Clear Creek (= Oriskany) beds and Cape Girardeau lime- 

 stone in southwest Illinois. He concluded that 



"these limestones represent the same geological horizon as the Niagara dolo- 

 mites in the northern part of the State ; and that the difference in the specific 

 character of the fossils is entirely due to the changes in the oceanic condi- 

 tions under' which they were deposited, and not to the different ages of the 

 sediments themselves." 



In describing the geology of Calhoun and Pike counties, Illinois, in 

 1870, Worthen 10 considered the gray limestone (= Sexton Creek or Brass- 

 field) appearing in the lower part of the Mississippi Eiver bluff, from 

 near the town of Rockport south to Hamburg, as the equivalent of the 

 Niagara limestone in the northern part of the State. He regarded as the 

 basal member of the Niagara in this region the gray oolite and the over- 

 lying buff magnesian limestone outcropping a few miles below Hamburg 

 and that are now known to correspond to the Edgewood as developed 

 near Louisiana, Missouri. 



In his description of the geology of Will and Kankakee counties, in 



8 A. H. Worthen : Geological Survey of Illinois, vol. iii, 1868, pp. 20-32. 



A. H. Worthen : Proc. Am. Asso. Adv. Sci., vol. xix, 1870, p. 1 72. 



10 A. H. Worthen : Geological Survey of Illinois, vol. iv, 1870, pp. 6, 7, and 26. 



