612 A. W. GRABAU TYPES OF SEDIMENTARY OVERLAP 



by the resemblance of this basal shale to an old residual soil, though Hayes 

 believes that in some areas the old-land surface was scoured by ocean cur- 

 rents before the deposition of the shale, f Where detailed observation 

 has been made the shale is said to pass upward into the overlying beds. 

 Though Ulrich has marked an unconformity at the top of the shale 

 in central Tennessee, he has so far failed to substantiate it by evidence. 

 Fossils are found in this shale in the northern areas indicating that its 

 age is late Devonic. Conodonts and the spores of freshwater Ehizocarps 

 are among the most characteristic fossils found, and the former occur 

 in many southern exposures of the shale. Zittel and Eohen have clearly 

 shown that these organisms are referable to oesophageal teeth of annelids. 

 Such organisms are today very characteristic of the muds and sands 

 of shallow shores and lagoons. The few marine fossils found in the 

 southern exposures west of the Cumberland ridge are either inconclusive 

 as to the age of the shale in that region, or, as in the Noel shale of 

 Missouri and Arkansas, they mark the age as Mississippian. From all 

 this it appears that the Black shale of southern United States is a basal 

 deposit — a residual soil of an ancient peneplain, very fine and very car- 

 bonaceous, and the result in many places of the solution of calcareous 

 strata. This soil was worked over by the transgressing Mississippian 

 sea, which rearranged it, washed it from the higher points, and collected 

 it in greater thickness in the depressions of the old peneplain. As the 

 water deepened, deposition of calcareous shales or of limestones followed, 

 the transition being a perfect one — sometimes gradual, sometimes abrupt. 

 If the view that the Black shale is the shore deposit of the sea, which 

 farther out deposited calcareous strata, is not accepted, a serious diffi- 

 culty confronts us; for if we assume, with Ulrich and others, that after 

 the deposition of the Black shale the sea retreated, and then readvanced, 

 we must account for the absence at the base of the calcareous strata of 

 a shore facies; for, surely, if the strata were successively deposited one 

 by one, each later overlapping the preceding one, the point of contact 

 between these strata and the Black shale, which point, at the time of 

 deposition of that stratum must have been the shore, should show some 

 evidence of that fact in the coarser clastic character of the strata and in 

 their inclusion of some fragments of the Black shale surface of the old- 

 land. That no such evidence is found clearly proves that the Black 

 shale represents the shore facies of each succeeding limestone or calcareous 

 shale stratum, and that it is hence not of uniform age throughout, but 

 varies from place to place. If we accept this view — and there seems to 



t C. W. Hayes : The Tennessee phosphates, Seventeenth Ann. Rept. U. S. Geological 

 Survey, pt. vi, p. 610. 



