LIMESTONES OF WESTERN TENNESSEE 097 



Along Horse Creek, the Ordovician and Silurian rocks have been 

 reduced by Cretaceous erosion to a sort of base level and then 

 covered by the Coffee sand. The Helderbergian exposures at 

 the Calybeate Springs appear to have risen above the level of 

 the Coffee sand (Fig. 3, p. 579). No careful study of the Coffee 

 sand or of the iron-ore gravels has yet been made in the area in 

 question. 



H. THE AGE OF THE CINCINNATI GEANTICLINE. 



I 5. As determined by observations i?i Teimessee. — Along the Cin- 

 cinnati geanticline in Tennessee the Chattanooga black shale 

 rests unconformably on Silurian and Ordovician rocks. Along 

 the crest of the geanticline, for a width of 55 to 75 miles, the 

 Black shale, where not removed by subsequent erosion, rests 

 upon Ordovician strata. On either side of this area it rests upon 

 the Silurian. Along the eastern line of outcrop of the Silu- 

 rian, on the western flank of the geanticline, the Black shale 

 rests upon the Clinton. Farther westward, proceeding to points 

 more distant from the broad crest of the geanticline, the Black 

 shale rests successively upon the Osgood, Laurel, Waldron, and 

 Louisville beds, the higher subdivisions of the Silurian. On pro- 

 ceeding still farther westward, into the basin of the Tennessee 

 River, this unconformity continues, so that the Black shale rests 

 upon higher Silurian rocks in the Tennessee River valley than 

 at Pegram, Centreville, Riverside, or Iron City. It rests upon 

 higher layers of the Brownsport bed along the Tennessee River 

 than at the more eastern exposures of this bed. East of Linden 

 the Black shale rests upon the attenuated edge of the Linden 

 bed. At Linden it rests upon the still more attenuated edge of 

 the Camden chert. West of the Tennessee River both the Linden 

 and the Camden beds are represented by thicker sections. 



The Tennessee River region may be regarded as having 

 formed, in times preceding the deposition of the Black shale, a 

 broad geosyncline of Silurian and Devonian limestones, lying 

 west of a broad geanticline of Ordovician strata. The axis of 

 the geosyncline lay a considerable distance west of the Tennes- 

 see River. Both lithologically and paleontologically the Silu- 



