1913-] STEVENSON— FORMATION OF COAL BEDS. 51 



often so serious that great care must be taken in selection for the 

 manufactin^e of high grade fire-brick. A similar condition was ob- 

 served in mines working down the dip, the only difference being that 

 the effects of freezing and thawing were perceptible to a somewhat 

 greater distance. H. Ries has informed the writer that the effect of 

 weathering rarely extends beyond 15 feet in a horizontal bed of clay. 

 The source of the clays is not always clear. It is true that clay 

 is not always present under coal beds, for those rest indifferently on 

 clay, limestone, shale sandstone or conglomerate, just as modern 

 peat bogs do, so that for present purposes the question of source is 

 of subordinate importance. At the same time, it is not without in- 

 terest, for in a great proportion of cases, conditions favoring accu- 

 mulation of coal followed those favoring deposition of clays. 

 Firket's^' observations have been cited frequently as showing that 

 atmospheric water can convert shale into plastic clay and in support 

 of the suggestion that underclays may be due to changes after 

 deposit. Near Liege a shaft, 30 meters deep, reached an ancient 

 mine which had been abandoned probably 700 years before. There 

 the succession, descending, was Psammite, 0.95 m. ; Gray plastic 

 clay, 0.40 m. ; Shale, not measured. The clay is very similar to the 

 refractory clay of Ardenne. The psammite had given way, was 

 broken and atmospheric water was admitted, which gave to that rock 

 a brown tint while it changed the upper part of the shale into 

 refractory clay. At another locality, the psammite in ancient work- 

 ings had become sandy micaceous clay and the shale had become 

 converted into black clay. Firket concluded that, under some cir- 

 cumstances, shale rocks may undergo considerable alteration siir 

 place. The action of true mineral springs is not required to effect 

 change of shale into clay, but infiltration of pluvial waters pene- 

 trating the ground across a small thickness of rocks may have an 

 influence. It is unnecessary in that case to have the action extend 

 over a long period in order to change 0.40 meter of shale into plastic 

 clay, for not more than 700 years had passed since the ancient mines 

 were abandoned. 



^^A, Firket, "Transformation sur place du schiste houiller en argille 

 plastique," Ann. Soc. Geol. de Belgique, Vol. I., 1874, pp. 60-63. 



