42 STEVENSON— FORMATION OF COAL BEDS. [April i8, 



by Mammatt^^ after his study of the Ashby-de-la-Zouche basin. 

 Logan^*' reached the same conclusion independently, several years 

 afterward, as the result of studies in south Wales. His statements 

 led to a comparison of notes and the conditions seemed to be the 

 same everywhere. The relations of the Illinois coal beds have been 

 cited as evidence that the condition is by no means general, but the 

 citation is an error, for Worthen's^^ remarks are so clear that one is 

 at a loss to comprehend how the error came about. He says 



The typical fireclay, the " underclay," " seat," or "mur " is rather 

 fine in grain, somewhat sandy, very light gray to almost black, the 

 tint depending on presence or absence of vegetable matter. Car- 

 bonate of iron is almost invariably present, sometimes in very small 

 quantity but many times it is abundant in nodules. Alkalies are 

 comparatively unimportant, though often present in sufficient 

 quantity to unfit the material for firebrick. Ordinarily, the rock is 

 plastic, but occasionally it is hard and non-plastic, a " flint clay." 

 This clay seldom shows lamination and on exposure to the air it 

 breaks up quickly into irregular angular fragments. The remark- 

 able feature is the presence of Stigmaria, whose rhizomas are often 

 interlaced in very complex manner. Owing to the abundance of 

 the plants, the clay is often termed Stigniaria-clay ; but the presence 

 of that plant is not essential; where Sigillaria and Lepidodendron 

 are wanting or of rare occurrence, Stigmaria is absent. It has not 

 been reported from underclays of the Monongahela or higher forma- 

 tions in the Appalachian basin. 



The " coal-seat " is not always clay or even impure sandy clay. 



" E. Mammatt, " Coal Field of Ashby-de-la-Zouche," 1834, p. 7;^. 



" W. E. Logan, " On the Character of the Beds of Clay, Lying Imme- 

 diately Below the Coal Seams of South Wales," Proc. Geol. Soc. Loud., Vol. 

 III., pp. 27s, 276. 



" A. H. Worthen, Geol. Surv. Illinois, Vol. I., 1866, p. 59. 



" The coal seams are usually underlaid by a bed of fireclay, which varies 

 in thickness from a few inches to ten or twelve feet. This was the original 

 soil on which the vegetation that formed the coal grew, and it is often pene- 

 trated by the rootlets of the ancient Carboniferous trees, whose trunks and 

 branches have contributed to form the coal." 



