THE CHESTER SERIES IN ILLINOIS 397 



The siKceous limestone fades of the Vienna has been recognized 

 from eastern Pope County, across Pope and Johnson counties, 

 and into the eastern part of Union County, except where the forma- 

 tions are interrupted by faulting. The black shale facies of the 

 same formation probably extends eastward beyond Pope County 

 into Hardin. It has been recognized considerably farther west in 

 Union County than the hmestone, but it is not unlikely that the 

 limestone will be found to extend farther in that direction when 

 the detailed, mapping is carried across Union County. 



North of Golconda, in Pope County, a measured section through 

 the Vienna formation gives a thickness of seventy feet. This is 

 perhaps the maximum thickness that is known, and the average 

 thickness throughout the area occupied by the formation is prob- 

 ably about sixty feet, perhaps becoming considerably thinner 

 than this to the east. 



The fauna of the Vienna limestone has certain characteristics 

 which distinguish it from the other limestones of the Chester series. 

 The Prismopora so characteristic of the Glen Dean limestone is 

 present somewhat commonly in the Vienna in places, but it has 

 never been observed to be so abundant as it is in some localities in 

 the older formation. Associated with the Prismopora there is 

 found somewhat rarely the pelecypod species Sulcatopinna mis- 

 souriensis, which is highly characteristic of the next higher lime- 

 stone in the Chester series, the Menard. This notable mingling 

 of the Glen Dean and Menard species is perhaps the leading 

 characteristic of the fauna as a whole, although further studies of 

 the fauna must be made. 



In the absence of any representative of the Tar Springs sand- 

 stone in the Randolph County section, the determination of the 

 Vienna equivalent in the section, if there is such an equivalent, is 

 rendered somewhat difficult. There is no lithologic unit present 

 in Randolph County similar to the siliceous limestone of the 

 Vienna, and no fauna has been recognized in which the Glen Dean 

 and Menard forms are associated. There is, however, locally at 

 least, lying above the Okaw limestone and beneath the typical 

 Menard, a conspicuous black-shale horizon, which may be a 

 continuation of. the black shale of the Vienna in the southern 



