20 FLUOKSl'AK DEP08ITS OF SOUTHP^RN ILLINOIS. I bull. 255. 



strotion). Mr. Ulricli would class the upper 25 feet of strata as 

 above described with the succeeding formation rather than with the 

 Tullahoma. 



Thickness. — P]xact measurements of the thickness of the formation 

 are not available, but checked estimates, ]:>ased upon dip measure- 

 ments and barometer observations, place it at 200 to 225 feet. 



Distribution and 7'elations. — The Tullahoma outcrops in north- 

 Avestern Hardin County in a belt around the dome of Ohio shale 

 near Hicks. Its area of outcrop is limited in turn by a similar belt 

 of St. Louis limestone, the next succeeding formation. So far as 

 known the fornuition does not come to the surface elsewhere in these 

 counties, though it is quite possible that small areas are exposed in 

 the faulted region east of that represented on the Hicks special map. 

 In Kentucky, Mr. Ulrich has recognized the formation near Smith- 

 land and near Princeton. It rests with slight uncomformity upon 

 the Devonian shale below. The relations to the St. Louis are less 

 clear, and it can only be stated that there is no obvious unconformity 

 in the Hicks area. 



Name and correlation. — The name Tullahoma is derived from a 

 locality in Tennessee, where it Avas applied by Safford in 1900 to the 

 equivalent beds. The formation represents early Mississippian time, 

 including probably that Avithin Avhich the Kinderhook, Burlington, 

 and Keokuk formations Avere deposited along the Mississippi. It 

 shoAvs only general faunal relationships to these formations, and it 

 is impracticable to make detailed correlations with them. 



ST. LOUIS LIMESTONE. 



Character. — The St. Louis limestone is for the most part a fine- 

 grained, dark-colored rock, breaking Avith a sharp conchoidal frac- 

 ture. It is moderately thin bedded and apparently nonmagnesian. 

 It is characterized by the presence of abundant chert nodules, Avhich 

 are irregularly rounded and are most marked along bedding planes. 

 They frequently show a distinct concentric structure, and individual 

 nodules 3 to 4 inches in diameter are A^ery prominent and character- 

 istic on Aveathered or fractured surfaces. 



The St. Louis limestone is apparently exceptionally soluble, and 

 its areas of outcrop are marked by numerous sink holes, Avhich often 

 contain ponds of Avater. The irregular surface and large number of 

 ponds remind one strongly of a drift-covered surface. 



TJiickness. — A thickness of 105 feet of limestone is exposed at Jacks 

 Point on the Ohio, near Elizabeth. The beds haAC a Ioav dip, Avhich, 

 if uiaintained. would give a total of several hundred feet as the thick- 

 ness exposed along the Ohio, but of this there is no certainty. Mr. 



