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



In 1868 there was published a detailed report on the geology 

 of Union and Alexander counties,* based on the studies of 

 A. H. "Worthen and Henry Englemann. In this work the 

 divisions of the Ordovician remain unchanged, but the term 

 " Clear Creek limestone " is restricted to only that part of the 

 siliceous limestones which is correlated with the Oriskany 

 series of the Devonian. To the Silurian there is referred the 

 lower 250 feet of these deposits under the name Lower Hel- 

 derberg limestone. 



In this report Worthen referred the so-called Lower Hel- 

 derberg limestone to a horizon higher than that of the Niagara 

 dolomites in northern Illinois. In 1870 he reverted to his 

 earlier views and correlated these limestones with the Niagara 

 dolomites farther north,f explaining the difference in the 

 specific character of the fossils in the respective deposits as 

 " entirely due to the difference in the oceanic conditions under 

 which they were laid down and not to the different ages of the 

 sediments themselves." 



Since 1870 no careful study of the above mentioned beds 

 has been made until detailed work was taken up by the writer 

 during a part of .the summers of 1907 and 1908. In the col- 

 lection of fossils the exposed ledges were worked by layers, 

 or arbitrarily divided into zones from six inches to a very few 

 feet in thickness. The fossils from each of these layers or 

 zones were kept' separate in order to determine the vertical 

 range and the relative abundance of the different species. 

 This detailed manner of work has revealed the presence of a 

 surprising number of unconformities, some of which would 

 not have been detected by any marked change in lithology, or 

 by a less careful method of study. In a preliminary statement 

 of the results of this work, a general section of the deposits 

 has been given .J 



Conditions of Deposition. 



The strata under consideration were laid down in an 

 arm of the sea which had connection southward with the 

 Mexican gulf region along a depression now occupied by 

 the lower course of the Mississippi river. Up this embay- 

 ment the sea pulsated backward and forward. Through the 

 southward connection the successive faunas reached the part 

 of the basin under consideration and spread towards the north, 

 east and west, to a greater or less distance, with increasing or 

 decreasing depth of the water. A short distance to the west of 



* Worthen : Geol. Surv. 111., vol. iii, p. 20 et seq. 

 f Worthen : Proc. Am. Assoc. Adv. Sci., vol. xix, pp. 172-175. 

 X Savage, this Journal, vol. xxv, pp. 431-443, 1908 ; also, 111. State Geol. 

 Surv., Bull. No. 8, pp. 103-117, 1908. 



