PROGRESSIVE OVERLAP 601 



priori grounds considered by Ulrich to be Devonic, and is followed by the 

 Saint Louis limestone series. Of course if we regard the black shale as 

 Devonic, then the fact that it grades up into the Tullahoma indicates 

 that the base of the Tullahoma is basal Mississippian ; but until more 

 convincing evidence is brought forth of the Devonic age of the black 

 shale and the basal Mississippian age of the Tullahoma, the more normal 

 interpretation is that both are above the base of the Mississippian — ap- 

 proximately of upper Kinderhook or lower Osage age. 



Foerste has recorded the frequent presence of sandy and earthy layers 

 at the base of the Black shale in the most eastern exposures on the Cum- 

 berland river, these layers being phosphatic* This basal portion usually 

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

 locally. Sometimes it is replaced wholly or in part by greenish more 

 clayey layers. In this sandy layer occur weathered out fossils of the 

 underlying Ordovicic rocks. Foerste has also recorded traces of Chonetes 

 and other fossils in the fine grained rock immediately above the Black 

 shale, south of Kockdale.f He considers that the beds containing these 

 fossils may be of Waverly age. In regard to the contact between the 

 black shale and the Waverly, he says : 



"In the gulley southeast of the Oliver Williams house the base of the section 

 consists of a dark, sandy, partly conglomeratic rock 18 inches thick. Both the 

 Black shale and the phosphate rock are absent. Immediately above the con- 

 glomeratic rock occur 11 inches of light green clayey rock containing purple 

 brown phosphatic material, both in the form of small irregular particles and 

 of nodules. Above this are found 8 inches of crinoidal greenish rock, with 

 fish teeth. At the 'Big hill,' immediately westward, on the road to Waynesboro, 

 the entire Black shale section is absent. 



"The purple brown phosphatic material found immediately above the con- 

 glomeratic, sandy rock at the Oliver Williams locality resembles the material 

 forming the phosphatic nodules at the top of the Black Shale section in most 

 parts of Tennessee and Kentucky. The fish teeth appear to belong to the same 

 formation as the bed from which the phosphatic material was obtained. The 

 greenish clay material, however, belongs to the Waverly horizon, so that the 

 base of the Waverly appears to contain material derived from the eroded top 

 of the Black Shale bed. The crinoidal material is unquestionably of Waverly 

 age. The fine grained but not fissile rock in the old Sawmill hollow may also 

 be of Waverly age, since species of Chonetes of the same general form are 

 rather common at the base of the Waverly section in the northern part of 

 Giles county. The dark color of the rock may be due to the carbonaceous 

 material received from the denuded Black shale of this area, while the more 

 sandy character may be due to material washed in from some other source by 

 the Waverly sea. The gradual passage of the black rock upward into the 



* A. F. Foerste : Silurian and Devonian limestones of Tennessee and Kentucky. Bull. 

 Geol. Soc. Am., vol. 12, 1901, p. 427. 

 tLoc. cit, p. 428. 



