DEVONIAN FORMATIONS 427 



thick. At some points in these counties the Black shale is entirely 

 absent. One of these localities is that mentioned by Professor Safford : ^ 



"At the foot of the 'big hill,' five miles from (southwest of) Mount Pleasant, on 

 the Waynesboro road. Here the siliceous cherty layers [Waverly] rest directly on 

 the Nashville rocks [OrdovicianJ, and all intervening formations being absent." 



Sandy and earthy layers at base of Black shale. — At the most eastern 

 exposures on the Cumberland river, in Kentucky, a 3-inch layer of sandy 

 rock occurs at the base of the Black Shale section. This layer may be 

 traced Avestward down the river, and is phosphatic. Between Rowena 

 and Creelsboro that part of the formation which immediately overlies 

 the basal sandy layer is more earthy and less shaly. Its thickness 

 varies usually between 2 and 3 feet, but thicknesses of even 6 feet are 

 locally recorded. Instead of this brownish rock, a greenish, more" shaly 

 rock appears at various points between Swan Bottom and Burks ville. 

 Sometimes only the lower part of the brownish rock is replaced by the 

 greenish, more clayey layers. Since both the brownish rock and the 

 greenish, more clayey rock weather away more readily than the over- 

 lying Black shale, the base of the Black Shale sections exposed by the 

 steep cliffs along the Cumberland usually recedes a little. 



In Tennessee both the sandy layer and the brown, more earthy layers 

 at the base of the Black Shale section occur. The brown layers, break- 

 ing up into irregular fragments, are well seen in the South Tunnel and 

 Pegram sections. The sandy layer is seen also near John Suell's house, 

 west of Whites Bend, in the quarry near the road. Along the hillside 

 southwest of Newsom station this sandy layer is fossiliferous. At many 

 points in Maury county it contains minute gasteropods resembling 

 forms (Microceras and Cyclora) found in Ordovician rocks. 



Phosphatic character of base of Black shale. — At many points south of 

 the Cumberland river, a dark blue, fine grained, very phosphatic rock 

 takes the place of part or all of the brownish rock occurring at the base 

 of the Black shale in the Cumberland River sections of Kentucky. At 

 the bridge west of Pegram there are only 8 feet of the typical fissile 

 black shales, and these are overlaid by the nodules which mark the top 

 of the Black Shale formation at so many points in Kentucky and Ten- 

 nessee. Beneath these shales are found 11 feet of more earthy rock, 

 breaking up into irregular fragments. At the base are 14 inches of dark, 

 fine grained phosphatic rock — the " black phosphate " of southern Ten- 

 nessee.f This is the most northern recorded occurrence of the typical 

 phosphate rock. It contains numerous specimens of Lingida spatidaia. 



* Page 332. 



t Charles Willard Hayes : The Tennessee Phosphates. 17th Ann. Rept. U. S. Geol. Survey, 1896. 



LXI— Bull. Geol. Soc. Am., Vol. 12, 1900 



