334 Foerste — Ordovician-Silurian Contact in the 



direction, on passing from Kentucky toward Indiana, there is 

 an increase of thickness and the observations so far made 

 indicate that this increase continues northward from Madison 

 toward the areas in the central part of Ripley and Jennings 

 counties, in which the Clinton is absent. The two or three feet 

 of fossil if erons strata at the top of the Madison bed at Madison 

 appear to be the attenuated representatives of the much thicker 

 fossiliferous beds at the top of the Madison bed in the central 

 part of Ripley and Jennings counties. The Silurian, therefore, 

 instead of resting upon lower beds of Richmond age in the 

 region in which the Clinton is absent, actually rests on beds 

 representing the latest deposits of Madison age so far studied. 

 The small degree of unconformity between the top of the 

 Ordovician and the base of the Silurian in the areas of Indiana, 

 Ohio, and northern Kentucky, so far under investigation, is a 

 most striking feature, considering the great lapse of time inter- 

 vening between their disposition, as indicated by the great 

 difference of their faunas. 



The Lophospira hammelli-Holopea hubhardi fauna may be 

 traced from the southern margin of Ripley county to Floyds- 

 burg, in Kentucky, about 13 miles southeast of Charlestown 

 landing. The fauna appears to be a depauperate one, a num- 

 ber of genera being represented by species of comparatively 

 small size. 



Parallelism of certain features shown by Clinton on oppo- 

 site sides of the geanticline. — Along the same line of exposures, 

 between the northwestern corner oi Switzerland county and 

 Lagrange, in Kentucky, the Clinton is usually quite thin, has 

 a light red or pink instead of a salmon-brown color, and litho- 

 logically appears to be a dense siliceous limestone instead of a 

 detrital limestone. Salmon-brown detrital phases are seen 

 along this line but are less common. The area in which the 

 siliceous phases of the Clinton are common is indicated on 

 the accompanying map. Farther south, toward Mount Wash- 

 ington and for a short distance south of Salt river, in Kentucky, 

 the salmon-brown, detrital phases of the Clinton are seen again. 

 Northward, the salmon-brown phases of the Clinton extend as 

 far as Laurel, Indiana. 



The salmon-brown color of the Clinton in Indiana and 

 western Kentucky appears to be due to the presence in very 

 small quantity of some iron compound, sufficient to stain the 

 calcite, but not visible to the eye, even under a microscope, as 

 a substance distinct from the calcite. In Ohio and Kentucky, 

 east of the Cincinnati geanticline, ferruginous layers occur at 

 the top of the Clinton at many localities * within an area which 



*Ohio Geol. Surv., iii, p. 442, 1878. Ohio Geol. Surv., Rept. for 1870, pp. 

 268, 269. Kentucky Geol. Surv., Eept. on Bath county, by W. N. Linney, 

 p. 18, 1886. On the Clinton oolitic iron ores, this Journal, xli, 28, 1891. 



