396 STUART WELLER 



the west, and in western Johnson County there are places where 

 it is apparently not more than forty feet. In tracing the horizon 

 into the Mississippi Valley counties, the Tar Springs disappears 

 entirely, there being not even such a thin discontinuous sandstone 

 horizon as that which represents the Hardinsburg, the position of 

 the sandstone being occupied by a plane of unconformity above 

 the Upper Okaw or Glen Dean limestone. 



Vienna limestone. — The Vienna limestone is named from Vienna, 

 Johnson County, Illinois, where the formation is exposed in some 

 of the streets of the town, and in an old quarry just west of the 

 town. As it is commonly exposed, the Vienna exhibits two 

 rather distinct facies, an exceedingly siHceous limestone in the lower 

 portion of the formation, and a shale member above. In places 

 the limestone seems to comprise the greater portion of the forma- 

 tion, but in other localities it is mostly all shale, with but little of 

 the limestone present. As already stated the limestone is remark- 

 ably siliceous. The silica is in part in the form of chert layers, 

 and in part is finely disseminated through the limestone. On 

 weathering, the limestone with the disseminated silica loses the 

 lime by solution, and the residuum is a light, yellowish-brown, 

 porous rock, which on first inspection resembles a fine-grained 

 sandstone, but which contains no sand grains whatever. The 

 chert of the formation is quite persistent in character, the beds 

 commonly being from one to four inches thick and quite regular. 

 Near the surface the upper and lower portions of these chert 

 beds have been subjected to some decomposition and are much 

 Hghter in color for a fraction of an inch in depth than is the deep 

 chocolate brown of the central portion of the beds. As the lime- 

 stone with the chert layers is removed by weathering, the cherts 

 fracture into subcubical masses with two light-colored surfaces, 

 and occur in abundance in the residuum. Less commonly there 

 are some thicker chert beds, six to eight inches, which are more or 

 less porous in character. The peculiar character of the weathered 

 products of the Vienna limestone make it about the easiest to 

 recognize of any of the limestone formations of the whole Chester 

 series. The shales of the Vienna are black, fissile, and non- 

 calcareous. 



