606 A. W. GRABATJ TYPES OF SEDIMENTARY OVERLAP 



Except where an erosion interval is responsible for the thinning away 

 of the Fort Payne or Siliceous group, we must assume that the thinning 

 is due to the failure of the basal members which are overlapped by the 

 higher members. All the descriptions indicate a passage of the Fort 

 Payne into the overlying lower Bangor, or its equivalent, the Floyd 

 shale. Where the Fort Payne is represented by less than 100 feet, this 

 thickness must represent the upper, or Tuscumbia, portion, unless an 

 erosion interval has removed a part of the upper portion before the depo- 

 sition of the Bangor. As already noted, however, such a disconformity 

 has not been recognized. It may, of course, be true that the Fort Payne, 

 recognized by its lithic character, has not the same position everywhere, 

 and that the lower Bangor of the southeast may represent a part of the 

 Tuscumbia farther west. However that may be — and the point can 

 only be settled by a careful examination of the sections — the fact remains 

 that the Lithostrotion bed in many places is close to the Black shale, 

 and in fact lies directly upon it. 



In the McMinnville quadrangle the base of the Fort Payne, which 

 varies from 150 to 225 feet, is probably of Keokuk age. 



Southeastward, in the Sequatchee valley, the Chattanooga (12 to 25 

 feet thick) is succeeded by only from 60 to 80 feet of the cherty Fort 

 Payne, which here represents the highest part of the group, or the Saint 

 Louis horizon, unless there is an erosion break at the top or the top is 

 here lower than elsewhere. It is described as passing upward into the 

 Bangor limestone.* 



In the type region, in the Tennessee valley, near Chattanooga, Ten- 

 nessee, the shale is from 10 to 25 feet thick, while the overlying Fort 

 Payne is reduced to from 60 to 150 feet in the western region and from 

 50 to 75 feet in the southeastern region. As before, if the section here 

 is complete, the Fort Payne can only represent the Saint Louis horizon. 

 Above it comes the Floyd shale in the eastern, nearer shore section, and 

 limestones in the more western, offshore district. The Floyd shale of the 

 eastern section is later replaced by limestone of the type of the Bangor, 

 but the equivalent of only the upper Bangor of the sections farther west, 

 the Floyd shale itself being the equivalent of the lower Bangor. 



A remarkable condition is found in the Chilowee mountain area. Here 

 the Chattanooga shale ranges from 6 to 30 feet in thickness, and is suc- 

 ceeded by the Grainger shale. It "comprises flaggy sandstones, sandy 

 shales and sandstones, with white sandstone and red and brown sandy 

 shales at the top; and this series is present throughout." f The lower 



* Sewanee folio. 



t Loudon and Knoxville folios. 



