Geography of Ohio 377 



place the boundary follows <*m iiTc^ular line south wai-d 11irouj;h 

 Xeiiia, near Wiliniiijiiou, and thence to tlie Ohio river. West- 

 ward iVom the (li'eat Miami, tlie boundary is very in-cfiular, 

 an arm of the Ordovieian rocks being exposed northward along 

 each of tlie more important tributai'ics of this rivcM-, among Ihefji 

 the Stillwater and Twin ei-ecks. Within the area thus described 

 are a few patch(>s of rocks belonging to the n(>xt later ])criod; 

 these are outliers or erosion I'emnants; in the southeast ])ait of 

 the n^gion, also, sti'eaiii cidting, chiefly that of Brush cre(>k, has 

 given the Ordovieian rocks an irregular surface distribut ion. 



Silurian, ''i'lu^ boundary of the area just desci-ibcd is the lower 

 contact line of the Silurian formations. From this line the Silur- 

 ian rocks extend to the east and north ^s far as J^ake J<]rie, and 

 into Michigan. The eastern boundary of the Silurian is (luite 

 regular fi'om Sandusky bay south to Jioss county; from I{(!ss 

 county to the Ohio river the margin of the outcrop is irregular, 

 a condition due to river work. Omitting the (h-divician area 

 and a region ifi tlie nortliwest part of the state, embraced by Wil- 

 liams, Fulton, ]3ehance, Henry, and slight ))ortioiis of adjacent 

 counties, all of the state west of this tiorth-south line belongs to 

 the Sihu'ian. The Bass islands, and all save tlie (Cistern, end of 

 the peninsula formed by Sandusky bay, are comprised within Sil- 

 urian territory. In Logan, county, east of Jiellefontaine, is an 

 area of rock belonging to the next later period; rocks of the same 

 period are on the surface also in Taylor township of Harden, county . 

 With these two exceptions, the territory descrilxMl belongs ent ii'cly 

 to the Silurian. For a distance of about ten miles along t he Ohio 

 river west of Vanceburg, Kentucky, the Silurian formations (jut- 

 crop. 



Devonian. The chief exposui'cs of the Dexonian locks in Ohio 

 border Lake Erie westward to Sandusky, and occupy a strip 

 about thirty miles wide extending south across the state as far 

 as ( 'hillicothe. From this ])oint to the Ohio river, the boundary 

 of the Devonian outcrop becomes very irregular and decreases 

 in width; it shows along the Ohio river for about twenty miles 

 east of Van.cel)urg. Along the Scioto valley, southward from 

 Waverly almost to Lucasville, the Devonian is on the surface; 

 the same outcrop continues westward in a narrow band through 

 Pike county. From the western side of ]*ike county southward 

 through Adams and Scioto count ies the rocks of this ])eri()d have 



