T. P. Savage — Ordovioian and Silurian Formations. 515 



lithology, and in the fauna, in passing from the Thebes sand- 

 stone to the succeeding deposits. It is shown in a strongly 

 weathered and iron-stained zone at the top of the Thebes 

 formation ; and in the fact that in different exposures the suc- 

 ceeding deposits rest upon different levels of the Thebes sand- 

 stone. 



The Orchard Creek Shale. 



The name Orchard Creek shale is here proposed for a bed 

 (=2b of my generalized section of 1908) of calcareous shale 

 exposed in the banks of Orchard creek, about two miles south of 

 Thebes. The formation is embraced between the Thebes sand- 

 stone below and the Girardeau limestone above. The material 

 consists of bands of bluish-gray shale, four to six inches thick, 

 alternating with two- to four-inch layers of impure, concre- 

 tionary limestone. The maximum thickness of the bed is 

 about twenty-two feet. 



An exposure of this shale, underlying the Girardeau lime- 

 stone, may be seen near the mouth of Orchard creek above 

 mentioned. It appears, above the Thebes sandstone, along the 

 bank of the river, and in the cut along the Chicago and East- 

 ern Illinois railroad, between Thebes and the village of Gale. 



The more characteristic fossils of this horizon are Cyclo- 

 cystoides cf. illinoisensis, Phylloporina granistriata, Dal- 

 manella meeki, JLeptwna rhomboidalis, Rajinesquina alternata, 

 Rhynchotrema cf. incequivalve, Strophomena near i?)curvata, 

 Zygospira recurvirostra % Cornulites tenuistriata, Conradella, 

 imoricata, Pterinea thebesensis and Isotelus sp. 



More than one-half of the species certainly identified from 

 this formation have not been reported from any other locality. 

 A number of them are recurrent Mohawkian forms. The 

 fauna lacks the characteristic Richmond fossils, but the pres- 

 ence of earlier types is not unusual in the Richmond strata. 

 The position of these beds, above the Fernvale and the Thebes 

 sandstone, refers the horizon certainly to the higher Richmond. 



The Post- Orchard Creek Disconformity. 



No well marked line of unconformity separates the Orchard 

 Creek shale from the overlying Girardeau limestone. How- 

 ever, such a sedimentary break is indicated by the great differ- 

 ence between the faunas of the two formations. Out of 

 sixteen species collected from the Orchard Creek shale and 

 twenty-seven species from the Girardeau limestone, only three 

 are common to the two horizons. These are Leptoe7ia rhom- 

 hoidalis, Cornulites tenuistriala and Pterinea thebesensis, all 

 long-ranging species. This almost total change in the fossils, 

 accompanied by no abrupt change in the lithology, is con- 



