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



The Devonian limestone in the Harpeth River region is usually very 

 white and crystalline. At the bridge west of Pegram this white lime- 

 stone is overlaid by 6 to 12 inches of dark earthy rock, and above this 

 lies the Black shale. The white limestone seems to pass gradually into 

 this earthy rock. Camarotcechia Carolina, Tropidoleptus carinatus, and 

 Spirifer pennatus have so far been found only in the earthy layer. Nucleo- 

 crinns verneuli occurs at various levels in the white limestone. The other 

 fossils mentioned in the list given at the close of this paper (see page 

 437) are found near the top of the white limestone. As far as may be 

 determined at present, the top of the Pegram limestone corresponds to 

 the Sellersburg limestone of Indiana, and to the Hamilton of New York, 

 while the lower part indicates at least one characteristic Corniferous fossil. 



In central and northern Kentuck3' the Devonian limestone rests on 

 successively higher subdivisions of the Silurian, on ascending the flanks 

 of the anticline. In the Harpeth River region of Tennessee it is known 

 only where resting on the Louisville limestone, but it appears to rest on 

 lower beds of this limestone at Newsom and Sam Walker's than at the 

 bridge west of Pegram, as far as can be judged by the absence, at the 

 eastern localities, of the fossiliferous Louisville beds discovered near 

 the bridge. 



THE CHATTANOOGA BLACK SHALE 



Variations in thickness. — Although the Devonian limestone of Kehtucky 

 thins out and disappears a short distance south of the central part of 

 the state, an equivalent of this limestone is known in Tennessee, but it 

 has so far been found onl}'^ in one limited area — in the valley of the 

 Harpeth river. 



The Black shale, however, has a much more general distribution. 

 Although the Black shale also thins out southward, it disappears en- 

 tirely at but few points. 



Along the valley of the Cumberland river, between Mill Springs and 

 Burksville (figures 6 and 7), the Black shale decreases from 40 to less 

 tlmn 35 feet. West of Lafayette, in northern Tennessee, the thickness 

 varies usually between 25 and 30 feet, but the thickness is much less 

 constant here than east of Burksville. At the spring southeast of La- 

 fayette, for instance, the Black shale is only 13 feet thick, and west of 

 Whites Bend, as already noted, it is reduced to 4 feet, and at a number 

 of points is entirely absent. 



Along the Harpeth river, west of Newsom, the Black shale is 20 feet 

 thick. South of this river most exposures seem to vary between 5 and 8 

 feet. At numerous points in southern Maury county, in Lewis county, 

 and in northern Giles county the Black shale is only 12 to 15 inches 



