92 J. J. STEVENSON — LOWER CARBONIFEROUS, APPALACHIAN BASIN 



tucky, where, however, the available observations suffice for an approxi- 

 mation. 



A shale underlies the limestone in some portions of the basin. It ap- 

 pears to be persistent in the Anthracite strip, but in the Allegheny region 

 its presence is doubtful except in Lycoming county. It is absent in the 

 greater part of that region in Pennsylvania as well as under the Laurel 

 and Chestnut anticlinals ;but oil-well records prove its presence in west- 

 ern Penns3^1vania along a narrow strip southward from Butler county 

 almost to the West Virginia line. It is present in Pocahontas county of 

 West Virginia and is recorded occasionally in wells of that state. Usu- 

 ally it is overlapped b}^ the limestone, but in the strip within western 

 Pennsylvania it evidently extends farther north ; the shale may have 

 had greater eastward extension in the anthracite fields, but no positive 

 assertion can be made, for in those fields the results of the first and 

 second geological surveys are not wholly in accord respecting the lime- 

 stones or, better, the calcareous beds. 



It is very evident that at the close of the Logan the trough of sedi- 

 mentation was greatly contracted on the north and west, and that in 

 some portions of Pennsylvania, where the limestone is present, there 

 was dry land at the beginning of the Tuscumbia, for in a large area 

 within the central portion of the state the shales are wanting. Local 

 foldings of slight extent must have been very numerous, as shown by 

 the absence of the shales in so mau}^ localities within West Virginia. 



The most northerl}^ point of the basin during deposition of the lime- 

 stone was apparently midwa}^ in the northern anthracite field. The 

 rate of subsidence increased southwestwardly, as iu that direction the 

 thickness increases, being greatest in the Cumberland Plateau region — 

 equivalent closel}^ to the Alleghenies of Pennsylvania, which shows also 

 that as in the Logan the axis of greatest subsidence lay somewhat west 

 from that for the Devonian. Whether or not the encroachment of sea 

 on the land continued at the southeast can not be ascertained at present. 

 Tuscumbia is clearly present in Calhoun county, Alabama, within 3 or 4 

 miles of the extreme Fort Payne outcrop at the southeast. Tuscumbia 

 appears to be recognizable at the extreme southern exposures in central 

 Alabama. 



Almost as far south as Tennessee, in the eastern and middle portions of 

 the basin, the Tuscumbia limestone is very* arenaceous, weathering to a 

 sand. Fossils are extremely rare, but those which have been found are 

 marine. The continuity of the calcareous deposit was interrupted for a 

 short time, during which a sandstone, ver}^ coarse at one locality but 

 ordinarily very fine, was laid down over a great area. The peculiar 

 current bedding of the limestone, thoroughly characteristic, is suggestive 



