THE 



AMERICAN JOURNAL OF SCIENCE 



[FOURTH SERIES.] 



Art. XXXVI. — The Ordovician-Silurian Contact in the 

 Ripley Island Area of Southern Indiana, ivith notes on the 

 age of the Cincinnati geanticline / by Aug. F. Foeeste. 

 (With Plate XVII.) 



Cincinnati Geanticline. 



The axis of the Cincinnati geanticline, a low fold, extends 

 from the northern part of Alabama through central Tennessee 

 and Kentucky into southwestern Ohio and the adjacent part of 

 Indiana. Northward it branches, one axis reaching the western 

 end of Lake Erie,* the other extending northwestward across 

 the northern half of Indian af into northern Illinois. In Ten- 

 nessee the rocks dip southeastward as far as Walden Ridge and 

 Cumberland Mountain, a distance varying from 75 to 100 

 miles ; on the western side of the axis the dip is westward or 

 northwestward at least as far as the Tennessee river, a distance 

 of 100 miles ; farther west, the Tertiary deposits conceal the 

 Paleozoic rocks involved in the fold. In the southern half of 

 Kentucky the rocks dip westward for a distance of at least 100 

 miles. From the northern half of Kentucky the dip is west- 

 ward across the state of Indiana as far as the eastern part of 

 Illinois, a distance approximately of 200 miles. Northward, 

 the rocks dip westward from the western branch of the axis 

 across Indiana and the eastern third of Illinois. Eastward, the 

 rocks dip from the eastern branch of the axis across the state 

 of Ohio as far as western Pennsylvania and the western part 

 of West Yirginia, the maximum distance being at least 180 

 miles. 



Over the greater part of the area the dips are too low to be 

 detected from any single point of view. As a rule, however, a 

 comparison of levels of corresponding strata from exposures 



* Ohio. Geol. Surv., vi, 1888, map opposite p. 48. 

 f Indiana Geol. Surv., 18th Kept., 1894, p. 221. 



Am. Jour. Sci.— Fourth Series, Yol. XVIII, No. 107.— November, 1904 

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