EROSION AT SILURIAN-DEVONIAN CONTACT 431 



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

 terial included 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 de- 

 rived from the underlying formations. The sandy material itself is prob- 

 ably of residual origin. It may represent a residual soil, but the evi- 

 dence 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 ex- 

 cept at their base, and their remarkably wide geographical distribution 

 suggest that they may possibly consist of wind-blown particles, derived 

 perhaps from many strata, from points far distant from one another. 

 The absence of coarser detrital material suggests that the region of depo- 

 sition was practically flat. The preservation of fragments of land plants 

 indicates that it was probably a region of marshes. It may be imagined 

 that the same particles traveled in many directions before finding a final 

 lodgment. Marshes at one point may have dried up and the material 

 accumulated in it may have again turned into dust, thus permitting the 

 frequent shifting by the wind of the materials which now form the shale. 



While all this may be readily enough imagined, it must not be for- 

 gotten that very few facts have so far been collected which have any 

 bearing upon the solution of the problems involved. 



Ordovician — Richmond Group 



general relationships 



Observations on Silurian rocks so far made in Kentucky and Ten- 

 nessee indicate that the Cincinnati anticline was in existence before the 

 deposition of the Devonian limestone (Corniferous and Hamilton) and 

 of the Black shale (Genesee and Portage), but do not give indubitable 

 evidence of the existence of this fold in early Silurian times. It has 

 been believed by some that the conglomerate in the Clinton near Belfast, 

 in Highland county, Ohio, indicates the presence of this fold, or at least 

 a beginning of its development, in late Ordovician or in early Silurian 

 times. At one time the materials of this conglomerate were believed to 

 have been derived from the Ordovician, but it is now known that they 

 were derived from the immediately underlying beds belonging to the 

 middle and upper parts of the Clinton. 



It is possible that a detailed study of the Ordovician formations in 

 the areas now occupied by the Cincinnati anticline may demonstrate a 

 development of this fold in times even as early as the Ordovician. So 



