408 A. F. FOERSTE — LIMESTONES OF TENNESSEE AND KENTUCKY 



At Bledsoe (locality 3) only the harder layers of limestone are well 

 exposed, the softer layers have disintegrated and have allowed the better 

 preserved layers to drop. It is evident that at one time the section must 

 have consisted of fine-grained, white limestone ; harder, more calcareous 

 layers being interbedded with softer, more argillaceous beds. At Newsom 

 (locality 12) the exposures are poor, due, no doubt, to the presence of 

 readily weathered beds. At the bridge west of Pegram (locality 9) the 

 upper 10 feet of the section are occupied by calcareous clay (plate 41, 

 figure 1) containing numerous fossils. The limestone beneath, above 

 the level of the railroad track, is lithologically ver}^ similar to the poorer 

 quality of limestone at Louisville. Underlying this rather soft limestone, 

 below the level of the railroad, are soft, clayey beds, and, still lower, in 

 the bed of the wet weather stream toward the north, there is a series of 

 hard limestone layers, containing Spirifer foggi and other characteristic 

 Louisville limestone fossils. 



The Louisville limestone is known at present only where unconform- 

 ably overlaid b}' the Black shale. Since it is probable that erosion took 

 place jirevious to the deposition of the Black shale, it is not possible to 

 form even an idea of the original thickness of this formation. The 

 thickest section measured in Tennessee is that at Bledsoe, 82^ feet. 



THE SILURIAN-DEVONIAN UNCONFORMITY 



The Silurian formations in Tennessee are overlaid unconformably by 

 the Devonian. The evidence for this is abundant (figures 3, 4, and 5). 

 The determination of the precise character of this unconformity was 

 made possible by the recognition of the various subdivisions of the 

 Silurian, as defined above. 



At South Tunnel (figure 1, locality 4) the Chattanooga Black shale 

 (Devonian) rests on only 5 feet of A\'aldron clay shale. The upper part 

 of the latter bed was probably removed before the deposition of the 

 Black shale. At Bledsoe (localit}" 3) the Black shale rests on at least 

 82 feet of Louisville limestone. At the home of S. R. Wood (localit}' 2) 

 the Black shale rests on 18 2 feet of Laurel limestone. At the exposure 

 near Halum P. Weeks' house (locality 1) the base of the Black shale is 

 separated by an interval of 18 feet from the highest exposed part of the 

 Osgood shale. The thickness of Osgood clay shale exposed is 11 feet. 

 At the Weeks locality, therefore, the Black shale rests on a layer 22 feet 

 lower in the scale than the la3'er on which it rests at the home of S. R. 

 Wood. Along the road leading southward down the hill from the Gap 

 of the Ridge the Black shale rests directly on the Ordovician. The 

 structure may be that of a syncline, with its trough near Bledsoe. At 

 Bledsoe the Black shale rests on the highest Silurian strata exposed 



