LIMESTONES OF WESTERN TENNESSEE 689 



usually absent ; one exposure, i foot thick, occurs on Tucker 

 branch, and another, 2 feet thick, several miles west of the mouth 

 of Green River. A trace of Black shale, not well exposed, was 

 seen in a gully on the west side of Florse Creek, southeast of the 

 home of Cab Blount. It is absent at Pyburn Bluff, opposite 

 White Sulphur Spring. Five miles southwest of Mount Pleasant, 

 at the Big Hill on the road to Waynesboro, the Black slate is 

 absent, but a mile southeast of the hill it attains a thickness of 2 

 feet. At Dodson station and at Lynnville, 15 miles southeast of 

 Mount Pleasant, it does not exceed 15 inches. At Iron City it 

 varies from 6 inches to nothing. 



From these and many other observations it may be seen that 

 the Black shale is either very thin or is entirely absent in the 

 southern parts of Maury, Lewis, and Perry counties, and in all 

 of Giles, Lawrence, Wayne, and Hardin counties. 



The Black shale occurs also on the western side of the 

 Tennessee River. It was struck in digging a well north of the 

 landing at Saltillo. A considerable quantity of Black shale was 

 struck northwest of the furnace at Brownsport Furnace. A 

 small exposure occurs near the bottom of a broad valley along 

 the road half-way between Dixon Spring and Perryville. It 

 probably occurs at other localities farther north. The scarcity 

 of exposures is probably chiefly due to removal by erosion 

 during Cretaceous and Tertiary times, the sands and gravels of 

 these ages resting directly on Helderbergian and Silurian rocks 

 in a large part of the area so far investigated. 



At the more northern exposures the Black shale is usually 

 underlaid by a sandy rock, i or 2 feet thick, and overlaid by 

 another layer, usually only several inches thick, characterized 

 by the presence of phosphatic nodules. Southward the sandy 

 layer becomes thicker, developing into the Hardin sandstone. 

 Owing to the thinning out of the Black shale in this direction, 

 the nodule layer not infrequently rests directly on the Hardin 

 sandstone. This is shown at a number of exposures along the 

 upper part of the Buffalo River, between Riverside and Flat- 

 woods. Farther south the phosphatic nodule layer is also 

 usually absent, so that in the southern counties the Hardin sand- 



