328 Foerste — Ordovician- Silurian Contact in the 



between the Silurian and Ordovician in this area. In seeking 

 to determine the measure of the erosion of Ordovician rocks in 

 times preceding the deposition of the Silurian in the Clinton- 

 less areas, it was expected that the Silurian would be found to 

 rest upon Ordovician rocks of a considerably lower horizon 

 where the Osgood rested directly on the Ordovician, than at 

 more remote points where the Clinton was present and the 

 pebbles were absent. ~No striking difference was noted. The 

 detailed observations are presented on the following pages. 



Top of Ordovician. — In Indiana, Ohio, and Kentucky, the 

 Cincinnati group at the top of the Ordovician is divided into 

 the Utica, Lorraine and Richmond, named in ascending order. 

 The Richmond is further subdivided into the Waynes ville, 

 Liberty, Whitewater, and Madison beds, also named in ascend- 

 ing order. The Madison bed has also been called the Saluda 

 bed, since the name Madison was employed earlier elsewhere. 

 The thickest exposures of the Madison bed are found on the 

 western side of the Cincinnati geanticline, along the Ohio river 

 and thence northward to Laurel, Indiana. 



In the neighborhood of Madison,* Indiana, a layer of mas- 

 sive corals, often of large size, has been adopted as marking the 

 base of the Madison bed. These corals are Columnaria 

 alveolata, Columnaria halli, and occasional specimens of Calo- 

 poecia crihriformis. Tetradium minus occurs 6 feet and, 

 again, 7*5 feet above the massive coral layer, sometimes also 2 

 feet below the same. Associated with the Tetradium, about 

 6 feet above the massive coral layer, are found Cyrtolites 

 ornatus, Bellerophon capax, Lopliospira howdeni, Ilormotoma 

 gracilis, Ischyrodonta miseneri, Pterinea demissa, and a small 

 form of Flaty strop Ma with a short hinge line. In the sandy 

 layer immediately above are found RhyncJtotrema capax, 

 Strophomena sidcata, Strophomena vetusta, Dinorthis sub- 

 quadrata, Hebertella sinuata, Streptelasma rusticum, Strep- 

 telasma divaricans, Beatricea undulatnm, and a large irregu- 

 lar lobate species of Heterospongia. As a rule fossils are scarce 

 at this horizon elsewhere in southern Indiana, with the exception 

 of Tetradiu?n, which is quite constant, and often very abundant, 

 at this level. 



The lower part of the Madison bed at Madison, 15 feet thick, 

 consists chiefly of clayey rock weathering readily. The greater 

 part of the remainder of the Madison bed, forming a section 32 

 feet thick, consists of a massive appearing more or less sandy 

 limestone, nearly unfossiliferous, often forming abrupt vertical 

 walls over which plunge the small streams leading to the Ohio. 

 This more massive rock oftens shows color-banding and is the 



* Indiana Geol. Snrv.. 21st Rept. 1897, pp. 248, 249; the lists of fossils by 

 Dr. Cornett and Prof. Cox are in error. 



