416 A. F. FOERSTE LIMESTONES OF TENNESSEE AND KENTUCKY 



22) and the Mrs R. C. Orman (locality 23) localities the rate of decrease 

 is calculated to be about 25 feet per mile. 



In contrast with the pronounced changes in thickness noted above, 

 where the Silurian formations are unconformabl}^ overlaid by the De- 

 vonian, note the comparatively small rate of change where the same 

 subdivisions are conformably overlaid by higher beds of the Silurian. 

 The greatest change in thickness noted in the case of the Clinton lime- 

 stone is that between Whites Bend (localit}^ 8) and Baker (locality 6), 

 a decrease of 12.5 feet in 15 miles, or less than 1 foot per mile. The 

 greatest change in thickness noted in the case of the Osgood bed is that 

 between South Tunnel (localit)^ 4) and Bledsoe (locality 3), a change 

 of 16.5 feet in 14 miles, or slightly more than 1 foot per mile. Usually 

 the rates of change per mile of the Silurian formations where overlaid 

 by other Silurian beds are much less than those mentioned above. 



EVIDENCE BY SILURTAN FORMATIOXS AS TO AGE OF CINCINNATI ANTICLINE 



Silaridn overlaid by Devonian beds. — The preceding observations make 

 it clear that along the western flank of the Cincinnati anticline the De- 

 vonian Black shale is deposited on the inclined margins of the various 

 subdivisions of the Silurian. In correlating the data obtained in Ten- 

 nessee and Kentucky it is evident that the Cincinnati anticline was con- 

 siderably develo])ed before the dei)osHion of the socalled Corniferous 

 limestone and of the Black shale. This brings up the question, What 

 relation has the thinning of the various subdivisions of the Silurian 

 along their margins on the western flank of the anticline to tiie history 

 of the latter's development ? 



In this connection it should be noted that the very rapid thinning of 

 the various Silurian formations is confined to those areas which are 

 nearest the axis of the anticline, where these beds are overlaid by the 

 Devonian. If the same formations be traced where they are overlaid by 

 other Silurian rocks, they are found either to be fairl}^ constant in thick- 

 ness or to vary at a much smaller rate. Moreover, in northern T-en- 

 nessee the Osgood and Laurel formations, when traced beneath other 

 Silurian beds, gradually increase in thickness toward the anticline for a 

 distance of many miles, but where overlaid by the Devonian they rapidly 

 become thinner, and disappear within a very short distance on being 

 followed in the same direction. 



These differences in the variations of thickness of Silurian formations 

 where overlaid b}'' the Silurian and where overlaid by the Devonian sug- 

 gest differences of histor3^ While those 2)arts of these formations which 

 are overlaid by other Silurian beds are probably preserved in the form 

 in which they were originally deposited, without any alteration in thick- 



