602 A. W. GRABATJ TYPES OF SEDIMENTARY OVERLAP 



greenish rock, as already described, is also favorable to the view that the black 

 rock, without good fissile cleavage, may be of Waverly origin. In case these 

 observations are correct, the absence of the Black shale at the 'Big hill' may be 

 due, not to original lack of deposition, but to subsequent erosion." 



Foerste raises the question whether the observed thinning of the 

 Black shale toward the Cincinnati dome may not be due to a "marked 

 development of the southern end of the Cincinnati anticline at the time 

 of the deposition of the Black shale and the base of the Waverly."* In 

 other words, he believes that less shale was deposited on the rising por- 

 tions of the dome. It may also be interpreted as resulting from the 

 washing of the residual soil from the higher into the lower places; and 

 this readily accounts for the absence of the shale in many places not as 

 due to erosion prior to the deposition of the Waverly and the occurrence 

 of a disconformity, which such a fact would imply, but as the result 

 of the washing of the original soil from the higher parts, on the en- 

 croachment of the lower Mississippian sea. This explanation is further 

 suggested by the fact that within short distances in all directions the shale 

 reappears. In general the coarseness of the material increases from the 

 northeast to the southwest. Kegarding the physical conditions under 

 which the shale was deposited, Foerste saysrf 



"In the case of the Black shale, the evidence of land conditions or of fresh- 

 water conditions is more favorable. At many points through its entire extent 

 it has retained remains of land plants. Its strongly carbonaceous character, 

 which gives rise to the black color of the shales, does not necessarily indicate 

 the presence of land plants, although the presumptive evidence is in favor of 

 this view. At various localities the remains of animals have been preserved 

 In this shale. . . . 



"The base of the Black shale is often decidedly earthy and is often also 

 phosphatic. It is well known that the base of the Black shale is in many 

 parts of southern Tennessee sufficiently phosphatic to be worked as a phos- 

 phate rock. One of the theories of the accumulation of the phosphatic ma- 

 terial at this horizon is that it was derived from the phosphatic material in- 

 cluded in the shells of the underlying Silurian and Ordovician rocks ; that it is 

 an accumulation in one sense of residual material. 



"This sandy base of the Black shale occasionally incloses fossils derived 

 from the underlying formations. The sandy material itself is probably of 

 residual origin. It may represent a residual soil, but the evidence is again 

 inconclusive. 



"The fissile black shale is composed of particles so light that they could have 

 easily been blown by the wind. The remarkably fine grained character of the 

 fissile shales, the entire absence of coarser material except at their base, and 

 their remarkably wide geographical distribution suggest that they may pos- 

 sibly consist of wind-blown particles, derived perhaps from many strata, from 



* Loc. cit., p. 429. 



f Loc. Cit., pp. 430, 431. 



