C— GEOLOGY. 59 



It may be pointed out that well-decomposed peat forms a buttery 

 mass almost, or perhaps quite, as impervious to water as a bed of clay 

 would be. This may explain why at Teversall Colliery there is a thin 

 bed of poor cannel at the base of the Top Hard (or Barnsley Bed) coal 

 and a second bed of better quality at the top. Where the lower bed 

 is thick the upper one is thin, and vice versd. 



Coal-Balls and their Significance. 



The bodies known, besides several aliases, as coal-balls are masses 

 of mixed vegetation ' petrified ' by being so completely permeated by 

 mineral substances, such as dolomite or calcite, that even the most 

 delicate and tender tissues have been preserved with every cell in its 

 proper position. Coal-balls occur in coal-seams as isolated masses, 

 varying in size from mere pellets up to masses of a ton or two in weight. 

 Sometimes they form clusters closely crowded together and at others 

 sporadically. Apart from their enormous value to palseobotany, they 

 present to the general practitioner in Coal-Measure Geology a number 

 of attractive problems, the solution of which cannot fail to throw a 

 vivid beam of light upoPx the question of the physiography of the coal- 

 swamps. 



Their limitation to seams carrying marine roof-measures at once 

 suggests a source for the petrifying substance and a reason why thej^ 

 are of such I'estricted occurrence that they are wholly unknown in the 

 great majority of coalfields. The notable memoir by Stopes and 

 Watson ^ is so important a compendium of the significant facts that 

 I shall forbear to cite the writings of others, including myself, wTio 

 contributed to the discussion. I would further say that I find myself 

 in almost complete agreement with the authors. Their argument in 

 brief is that the seams in question grew in salt or brackish swamps and 

 that a mass of debris of the plants accumulated under water. Sea- 

 water has a remarkable preservative effect upon plant-tissues, experi- 

 ments by one of the authors showing that fronds of ferns, and even the 

 more delicate structures of liverworts, could be preserved for at least 

 three years without signs of decay or even loss of their green colour. 

 They then proceed to argue that the partial decay of some vegetable 

 materials would liberate carbon dioxide which, reacting with sulphates 

 and sulphides with which the sea-water would have impregnated the 

 mass, produced these isolated concretions which represent a true sample 

 of a bed of peat accumulated on the spot where the plants grew. One 

 instance is cited of two seams separated by a sandstone seat-earth 

 (gannister) in which coal-balls are scattered through both seams. 

 Assuming, as the text implies, that the general character of the 

 concretions is the same throughout the sequence, the inference seems 

 to be justified that the formation of coal-balls was continuing during 

 the whole period of accumulation of the seam. At the same time, it is 

 not clear why the petrifaction should be so local, and it is perhaps worth 

 while to examine -any evidence which might decide whether, as happens 

 with some other rocks, the sporadic character may not be due to local 

 escape from decalcification rather than to local petrifaction. 



2 I'/iil. Tian-'i., Ser. B., vol. 200. 



