T. E. Savage — Stratigraphy of Southwestern Illinois. 437 



in Illinois, of about 237 feet. Fossils are somewhat rare in 

 the lower portion, but in the middle, and especially in the 

 upper, portion there is a rich fauna including Michelinia, n. 

 sp., Amboccelia cf. umbonata, Amphigenia curta, Anoplia 

 nucleata, Anoplotheea flabellites, A. fmbriata, Centronella 

 glansfagea, Chonostrophia reversa, Gyrtina hamiltonensis, 

 Eatonia peculiaris, E. cf. whitfieldi, Eodevonaria melonica, 

 JLeptostrophia perplana, Megalanteris condoni, Oriskania 

 sinuata n. var., Pholidops terminalis, Rhipidomella muscu- 

 losa, Spirifer worthenanus, S. duodenarius, S. macrothyris, 

 S. hemic ychos, S. tribulis, S. cf. murchisoni, Schuchertella 

 pandora, Acidaspis tuberculata, Odontocephalus arenarius 

 and Phacops cristata. 



These Upper Oriskany beds were deposited near the north 

 end of the Mississippian embayment, which at this time was 

 even more contracted than during the Helderbergian. The 

 basin was remote from, and not connected w T ith, the New 

 York-Maryland province. It covered western Kentucky and 

 Tennessee, and lapped over the southeast corner of Missouri 

 and the east side of Arkansas, spreading an arm across north- 

 ern Alabama. 



Onondaga. — The sedimentation of the Upper Oriskany time 

 continued without a break into the Onondaga or Corniferous. 

 The latter period was initiated by disturbances to the west- 

 ward, in Ozarkia, which increased mechanical sedimentation in 

 the Illinois area. These resulted for a time in the deposition, 

 along the eastern shore of Ozarkia, of layers of sand containing 

 Onondaga fossils alternating with the return of the Oriskanian 

 limestone conditions. Eventually sand deposition prevailed 

 and there was spread over the basin the basal sandstone of the 

 Onondaga formation (7a of section), containing Michelinia 

 stylopora, Aulacophyllum sp., Amphigenia curia, Centronella 

 glansfagea, Meristella near lentiformis, Rhipidomella muscu- 

 losa, Spirifer duodenarius, S. macrothyris, Conocardium 

 cuneus and Odontocephalus arenarius. 



Early in the Onondaga time an elevation in the southern 

 portion of Union and in Alexander county put a stop to 

 further deposition in these regions, while farther north, in 

 Jackson county, sedimentation was uninterrupted. 



At the cut through the Back Bone and at the Bake Oven, 

 a short distance north of Grand Tower, there is exposed a 

 continuous section of the Onondaga formation showing a thick- 

 ness of 115 feet. The beds consist largely of light colored, 

 regularly bedded, more or less crystalline limestone, which 

 becomes arenaceous in the lower part. Fossils are abundant 

 throughout the section. 



The upper layers are marked by Chonetes Ttonickianus, 

 Leptcena rhomhoidalis, Pentamerellaarata, P. papilionensis, 



