DEVONIAN FORMATIONS 425 



state definitely whether it is residual Ordovician material taken up into 

 the base of the Black shale or Devonian material poorly preserved. 



It probably, however, corresponds with the millstone rock of Professor 

 Safiford, and this is said by him to contain Nashville (Ordovician) 

 fossils. In that case it is probably residual material included in the 

 base of the Chattanooga Black shale. 



The following lines, copied from the report of Professor Safford,* give 

 his account of the millstone rock : 



"In Sumner county, a few miles north of Hartsville, immediately below the 

 Black shale, is a bed, from which millstones were formerly extensively manufact- 

 ured. This bed is a mass of shells, closely packed, and silicified. The bed is 

 several feet thick, and contains Nashville species. The shells are so packed as to 

 make the rock, in due degree, cellular. The weathered portions, near the out- 

 crop, ai"e preferred, for the reason that, within, the spaces between the shells are 

 filled with calcareous matter, which, by exposure, is leached out. The millstones 

 manufactured here, were highly esteemed. I do not know that any have been 

 made of late years (1869)." 



THE PEG RAM LIMESTONE 



In the valley of the Harpeth river (figures 1 and 2), between Newsom 

 and the bridge west of Pegram, all exposures of the Louisville formation 

 are overlaid by Devonian limestone — the Pegram limestone. 



Its thickness increases toward the west. At Newsom (locality 12) 

 it is 3 feet thick. Blocks of this stone occur west of the quarry nearest 

 the station, above a negro hut on the south face of the hill (plate 40, 

 figures 1 and 2). It occurs also three-quarters of a mile west of Foster 

 and Creighton switch, along the river southeast of Sam Walker's house. 

 Near the eastern end of the bluff, the Laurel, Waldron and Louisville 

 beds are well exposed. Overlying the latter is the Devonian limestone, 

 5 feet thick, covered at some points by 4 inches of sandstone or of sandy 

 limestone (locality 10). 



Possibly the 3 inches of sandy rock at the top of the Laurel lime- 

 stone south of Johnson V. Linton's house (locality 15) belong to this 

 horizon. 



At the bridge a mile and a half west of Pegram (locality 9) the De- 

 vonian limestone is 8 feet thick at the most eastern exposure along the 

 railroad (plate 41, figures 1 and 2). At its most western exposure, in 

 the quarry north of the railroad, the total thickness is 12 feet. The 

 upper 6 inches of the white Devonian limestone contain small black 

 pebbles, usually less than half an inch thick. Similar pebbles occur at 

 the top of the Devonian limestone in many parts of southern Indiana 

 and Kentucky. 



* Page 283. 



