GEOLOGICAL INTRODUCTION 23 



vieve age in the section, and below about 20 feet of limestone. This 

 sandstone is very fine-grained, yellowish, and very cross-bedded. It re- 

 sembles, in some respects, the Rosiclare sandstone of Hardin County; 

 and the section in which it occurs suggests the three-fold division of the 

 Ste. Genevieve of that county, as described by Ulrich. In Monroe County, 

 however, this bed does not appear to be a continuous formation, but only 

 a local phase of the formation which has become wholly arenaceous. 



The fauna of the Ste. Genevieve limestone is in part a recurrent 

 fauna from the Salem limestone, but associated with these recurrent 

 forms are certain others which are characteristic of the Ste. Genevieve. 

 The best index fossil of the formation is probably Pugnoides ottumwa, 

 originally described from the Pella beds of Iowa, which are to be corre- 

 lated with these Ste. Genevieve limestones. Another good index species, 

 but less common than the last, is Girtyella indianensis, which is also typical 

 of the Pella beds of Iowa. 



V. CHESTER GROUP 



The name Chester Group was originally applied by "Worth en to those 

 strata of the Mississippian series typically exposed in Randolph County, 

 from the base of the so-called "lower sandstone of the Chester group" 

 to the summit of the Mississippian. The maximum thickness of this 

 series of beds was estimated by Worthen to be at least 600 feet. A careful 

 measurement of the Randolph County section by the writer gives a total 

 thickness of about 550 feet, although a thickness of 743 feet is indicated 

 in the Gilster well at Chester. 



The local details of the Chester section vary from place to place, but the 

 larger features are persistent over wide areas. The lowermost sandstone 

 has long been recognized as a distinct formational unit and was called 

 Cypress sandstone by Henry Engelmann 1 from certain extensive ex- 

 posures on Cypress Creek in Johnson County. The series of beds super- 

 jacent to the Cypress was called the Kaskaskia limestone by Hall. More 

 recent field investigations have made it clear that the series must be 

 subdivided, and Ulrich, in his Kentucky work, 2 has defined two forma- 

 tions, the lower of which he calls Tribune limestone and the upper, Birds- 

 ville formation, which, together, constitute the Kaskaskia of Hall. How- 

 ever well this classification may apply to this series of beds in Ken- 

 tucky, it is inapplicable in the typical Illinois section, and it has been 

 found necessary, in the course of recent field work, to establish an entirely 

 new series of formational units for the Chester group. 



1 Trans. St. Louis Acad. Sci., vol. 2, pp. 189-190 (1863). 

 2U. S. Geol. Survey, Prof. Paper, No. 36, pp. 55-66 (1905). 



