516 T. E. Savage — Ordovician and Silurian Formations. 



sidered deciding evidence of a land interval between the time 

 of deposition of the respective beds. 



Silurian System — Alexandrian Series. 



The term Alexandrian Series has been proposed* to include 

 those strata which more or less completely bridge the interval 

 between the uppermost horizon of the Richmond and the 

 basal deposits of the Clinton. They carry faunas intermediate 

 in character between the Richmond and the Clinton, but not 

 distinctively those of either group. 



The formations in southern Illinois that contain faunas 

 which cannot properly be referred to the Richmond below, or 

 the Silurian above, but which have affinities in both directions, 

 are : 1, the Girardeau limestone, and 2, the Edgewood formation. 



The Girardeau Limestone. 



Strata of this age were first described by Shumard,f in 1855, 

 from Missouri under the name Cape Girardeau limestone. 

 Worthen recognized the horizon in southwest Illinois and 

 retained Shumard's name for the formation. 



The rocks consist of dark-colored, line-grained, compact, 

 brittle limestone, in imperfectly separating layers two to live 

 inches thick. Between the layers occur thin lenses of hard, 

 calcareous shale which locally contain numerous fossils. Among 

 the common species are Glyptocrinus fimbriatus, Tanaocri- 

 nus cf. typus, Camarotoechia scobina, Dalrnanella near 

 elegantula, Homoeospira sp., Leptama rhomhoidalis, JRafines- 

 quina mesacosta, Schicchertella miss ourien sis, Waldheimia (?) 

 bicarinata var., Cyclonema cancellata, Platyostoma niagar- 

 ensis var., Cyrtodonta primogenia, Pterinea thehesensis, 

 Acidaspis halli, Cyphaspis girardeauensis, Encrinurus del- 

 toideus and Lichas sp. 



The maximum thickness of the formation in Illinois is 

 about thirty-three feet. The strata are well exposed in the 

 banks of a creek two miles south of Thebes, and also along 

 the river one-half mile further south. North of Thebes they 

 outcrop along the Chicago and Eastern Illinois railroad, one- 

 half mile south of Gale. 



The Girardeau a transition fauna. — The fauna of the 

 Girardeau limestone, listed above, has a decidedly Silurian 

 aspect. The genera Homoeospira, Schuchertella, Waldheimia ? 

 and Platyostoma are distinctively Silurian, while not one of 

 the species could be considered a marker of a Richmond 

 horizon. A few of the Ordovician forms persist, but the 



* Savage : This Journal, vol. xxv, p. 434, May, 1908. 



f Shumard, B. F. : 1st and 2d Ann. Eepts. Geol. Surv. Mo., p. 109, 1855. 



