78 STEVENSON— FORMATION OF COAL BEDS. [April 18, 



Green'* in describing the Yorkshire roof balls says that the Gonia- 

 tites, Aviculopecten and other shells enclosed are not flattened as are 

 those in the shales. The plant material in the coal balls is in wholly 

 uncompressed condition, so that the minutest details of structure 

 can be recognized — as one may see by consulting Williamson's 

 memoirs in the Transactions of the Royal Society. Stur found the 

 stems of plants not only uncompressed but also, in some cases, not 

 wholly decayed, so that the concretions were formed before the 

 chemical change had been completed. Stopes and Watson were 

 convinced that they had traced a stem continuously from one coal 

 ball into another ; Wild says that the Lancashire " bullions," com- 

 posed of fossil wood, occasionally show rootlets working their way 

 through the decaying wood, separating the fibers which now sur- 

 round them. But vegetable fragments in roof balls are different ; 

 as Stur remarks, they are coaled and evidently much changed ; they 

 tell little of relations and less of structure. 



But coal balls are not confined to the Coal Measures. Gothan 

 having noted that the localities, where the balls had been obtained, 

 were all within paralic basins set himself to discover them under 

 other conditions. Petrified stems are common in Tertiary beds, 

 where, as deposition centers in brown coal, they have given oppor- 

 tunity for concentration. Such silicified or at times pyritized stems 

 occur frequently in the Halle brown coal and in the Rhenish brown 

 coal one finds the well-known oolite wood. But these are not wholly 

 analogous to coal balls, which are bits of petrified peat, penetrated at 

 times by roots of vegetation growing above. In searching the survey 

 collections at Berlin. Gothan found a piece of brown coal from the 

 Donatus mine near Cologne, which contained spherules of carbonate 

 of iron, the same as the material of the oolite wood. Deposition 

 had not been confined to the wood but had reached into the actual 

 peat. Specimens were procured from Fliigel, who had mapped the 

 area, and they proved to be part of the bed, replaced with material 

 like that of the plant-balls described by Stur. Gothan suggests the 

 name of Torf-Dolomite. Microscopic examination by Horich showed 

 the close resemblance between these forms and the coal balls. As a 



'*A. H. Green, "The Geology of the Yorkshire Coal Field," p. 108. 



