Ripley Island Area of Southern Indiana. 325 



sary to point out the inconclusiveness of this evidence founded 

 on observations at a single locality; however in view of opinions 

 as to the age of the Cincinnati geanticline then generally cur- 

 rent, the predication of the pre-Niagaran origin of the geanti- 

 cline on the basis of this Belfast exposure of conglomerate is 

 certainly comprehensible. Moreover, it is not impossible that 

 at some future time, in combination with other evidence not 

 yet at hand, it may prove to have an important bearing upon 

 the subject. 



More recent observations upon the Clinton of Indiana have 

 brought to light facts which are of interest in this connection. 



Ordovician- Silurian Contact in Ripley Island Area of South- 

 eastern Indiana. 



Base of Silurian. — When the investigation of Silurian 

 strata in Indiana was begun, the pre-Silurian origin of the Cin- 

 cinnati geanticline was accepted. In Ohio the Clinton was 

 known to become thinner westward. Near the Ohio river, in 

 Adams and Highland* counties, exposures equalling 35 feet, 

 in some instances possibly 40 feet, are seen. Northwestward, 

 in Clinton county, the Clinton has a thickness of at least 25, 

 possibly 30 feet. Still farther northwest, east of the Miami 

 river, in the northeastern part of Montgomery and the southeast- 

 ern part of Miami county, the thickness varies between 23 and 

 28 feet. Passing from the northern half of this northwesterly 

 running line of outcrop toward the southwest, the rate of 

 diminution of the Clinton is much more rapid. At Sunder- 

 land Falls and at Ludlow Falls the thickness is about 22 feet. 

 At Fauvers quarry, 2 miles north of Dayton, west of the 

 Stillwater river, and at Centre ville, 9 miles south of Dayton, 

 the thickness is 17 feet. At the Betty Heidy exposure, about 

 a mile east of Oregonia, the thickness is at least 14 feet, and 

 possibly equalled 16 feet. At the Soldiers' Home, west of 

 Dayton, it is about 15 feet. At Lewisburg and Eaton, the 

 thickness is 13 feet. East and southeast of Richmond in 

 Indiana it is about 1 1 feet. About 12 miles southwest of Eaton, 

 six miles east of the state line, the thickness does not appear to 

 exceed nine feet. 



The decrease in thickness of the Clinton toward the south- 

 west continues in Indiana.^ Glacial deposits cover the Clinton 

 in the western part of Wayne county, but in the western part 

 of Fayette and Franklin counties the thickness of the Clinton 

 usually does not exceed 6, but occasionally attains 8 feet, and 

 frequently is reduced to 4 feet or even less. Southward, across 

 the center of Ripley county, the thickness of the Clinton con- 



* An account cf Middle Silurian rocks of Ohio and Indiana, Journ. Cincin- 

 nati Soc. Nat. Hist., xviii, 1896. 



f Indiana Geol. Surv., 21st Rept. p. 213 ; and 22nd Kept. p. 195. 



