PAPERS BY MEMBERS 127 



beds of the Renault formation. The Ruma formation is the 

 highest formation in the Chester group in Randolph and 

 Monroe counties in which conspicuous variegated shale beds 

 have been observed. In at least one locality a thin lime- 

 stone ledge, one or two feet in thickness, has been observed 

 in the midst of the Ruma, but the occurrence of limestone in 

 the formation is rare- Excellent and typical exposures of the 

 Ruma formation may be seen in the stream valleys tributary 

 to Horse Creek, west and northwest of the village of Ruma. 



6. Okaw Formation. 



General characters. The Okaw formation comprises 

 a series of alternating limestones and shales which have an 

 aggregate thickness of from 150 to 200 feet. The valley of 

 the Kaskaskia or Okaw River is excavated through these 

 beds at its junction Avith the valley of the Mississippi, and 

 excellent exposures of the various members of the formation 

 are present both above and below this point in the Missis- 

 sippi River bluffs. 



At least four, and perhaps five important limestone 

 members are present in the formation, the uppermost of 

 which is the quarry ledge at the Penitentiary. These lime- 

 stones vary greatly in color from dark blue to gray and nearly 

 white, the weathered surfaces of some strata being buff or 

 brownish. The texture also varies from a very compact 

 limestone to more or less coarsely crystalline beds, with 

 some distinctly oolitic zones. One very persistent oolitic 

 bed nearly white in color, occurs about 50 or 60 feet from the 

 base of the formation. ]\Iost of the limestone beds are free 

 from chert, although some cherty horizons are present in the 

 formation. These calcareous members of the formation 

 sometimes consist of rather massive beds of limestone, but 

 more frequently the individual beds are a foot or less in 

 thickness, separated by thin shaly partings. 



The shale members of the Okaw formation, occu- 

 pying the intervals between the limestones, are soft and 

 easily acted upon by the weather, and consequently are rare- 

 ly well exposed. AMiere they have come under observation 

 in certain ravines, these shales are commonly blue or gray 

 in color, rarely with a slight admixture of red or purple, which 

 is such a conspicuous feature of some of the lower shale hori- 

 zons. 



The highest member of the Okaw, immediately 

 above the quarry ledge at the Penitentiary, consists of cal- 

 careous shales with interbedded thin limestones, and locally 

 a sandstone ledge 10 to 12 feet in thickness is present. 



