DEVONIAN. 281 



Generalized section of the pre- Mississippian strata in southwestern Illinois — Continued. 



Sys- 

 tem. 



■a 



o 



> 

 O 



1 



Correlations. 



Onondaga, 156f feet. 





Upper Oriskany — 

 Clear Creek chert, 

 Camden chert, 

 237 feet. 



Location of sections. 



Back Bone, Jack- 

 son County: 



Bake Oven, Jack- 

 son County. 



NW. i sec. 26, T. 

 12 S., R. 2 W., 

 alsoSW. Jsec. 26, 

 T. 13S.,R. 2W., 

 Union County. 



Schaffer's branch, 

 2 miles west of 

 Jonesboro; NW. i 

 sec. 26, T. 12 S., 

 R. 2 W., Union 

 County. 



NW.Jsec. 10,T. 12 

 S.,R.2W.;NE.J 

 sec. 12, T. 12 S., 

 R. 3 W., Union 

 County. NE. \ 

 sec. 36, T. 14 S., 

 R. 2 W., Alex- 

 ander County. 



Descriptions of horizons. 



Feet. 



The Onondaga is well developed in Jackson County, 

 where it passes without a break into the Hamilton. 

 In the southern part of Union County there is a 

 break and the Onondaga is represented only by 

 'the basal sandstone. 



Heavy layers of verjr hard gray, coarsely crystalline 

 limestone, containing corals, Chonetes konickianus, 

 Pholidostrophia iowensis, Productella spinulicosta, 

 and Stropheodonta concava. Strophalosia truncata 

 is abundant in the lower half, while Productella 

 spinulicosta is common in the upper part 26 



Layer of dark-colored limestone largely composed of 



Chonetes konickianus var 3§ 



Thin-bedded hard gray limestone, layers 2 to 10 

 inches thick. Fossils rare: Chonetes konickianus 

 var. present in the upper part and C. pusillus and 

 Stropheodonta concava m the lower 15 



Hard gray impure limestone with few fossils 21 



Dark-gray impure limestone with thin chert bands 

 near the top. Fossils numerous, Nucleocrinus 

 verneuili, Rhynchonella gainesi, Meristella barrisi, 

 Spirifer acuminatus, Stropheodonta patersoni, etc ... 8 J 



Dark-gray impure fine-grained limestone. Chonetes 

 mucronatus abundant in a zone near the middle. 

 Other fossils are Rhipidomella vamixemi, Spirifer 

 grieri, Stropheodonta patersoni, S. perplana, and 

 Phacops cristata 11 



Heavy layers of light-grajr subcrystalline limestone. 

 Fossils abundant. Cosdnium cribriformis, Centro- 

 nellaglansfagea, Spirifer diwdenarius, S. macrothyris, 

 and Odontocephalus segeria present throughout 38 



Alternating layers of light-gray arenaceous subcrys- 

 talline limestone and coarse-grained sandstone, 

 containing Centronella glansfagea, Meristella near 

 lentiformis, Rhipidomella musculosa, Spirifer duo- 

 denarius and 8. macrothyris 15^ 



Bed of more or less iron-stained sandstone, in places 

 soft and friable, at other points cemented by a 

 deposit of iron or silica, containing Michelinia 

 stylppora, Aulacophyllum sp., Centronella glans- 

 fagea, Spirifer duodenarius, 8. macrothyris and 

 Odontocephalus arenarius 18 



Bed of light-gray chert in layers 3 to 9 inches thick. 

 Arnphigenia curta, Chonostrophia reversa, JEodevon- 

 aria melonica, Schuchertella pandora, and Spirifer 

 worthenanus abundant 5 j 



Reddish-brown friable sandstone with Michelinia 

 stylopora, Zaphrentis sp., Amphigenia carta, and 

 Spirifer duodenarius ^ 25 



Layers of light chert, 4 to 8 inches thick. Anoplia 

 nucleata, Chonostrophia reversa, Eo^evonaria mel- 

 onica, Schuchertella pandora, and Spirifer worth- 

 enanus li 



R«ddish-brown friable sandstone 2 



Bed of light-colored chert layers, in places alternating 

 with impure siliceous limestone and at other points 

 composed wholly of chert bands. Fossils most 

 abundant in the upper part. Amphigenia curta, 

 Anoplotheca flabellites, Eatonia peculiaris, Eodevon- 

 aria melonica, Chonostrophia reversa, Schuchertella 

 pandora, Spirifer worthenanus, and S. hemicyclus 

 common in the upper part 225 



In the southern part of Union County the lower chert 

 layers are massive and contain but few fossils. In 

 the pit worked by the M. & O. Railroad, 1\ miles 

 north of Tamms, in Alexander County, may be 

 seen an exposure of more than 100 feet in which 

 few fossils were found. 



