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



sinking of dissolved humic and other organic acids, which reaching 

 the bottom may remove iron and alkalies from the clay as they do 

 from the peat. If the original quantity of iron in the mur was 

 small, all or practically all might be removed ; but if large, the greater 

 part would remain. In any event there would be a chemical change 

 and the color would become lighter, though enough iron might re- 

 main to become distinct after burning. 



The tinting of underclays depends in great measure on the quan- 

 tity of carbon present. Changes during conversion would remove 

 some vegetable matter, but not much, for drainage would be 

 chiefly along the surfaces of roots, which may account for the 

 lack of a coal crust, so often observed in Stigmaria. The removal 

 could not be extensive throughout the mass, so that if the original 

 quantity was considerable, the clay would be blackened. 



The suggestion has been made that gray or whitish murs are not 

 common and that the tint is not original, for, at some distance from 

 the outcrop, the color is not distinctive. The light-colored English 

 clays, it is stated, have been exploited only along the outcrop, where 

 the passage of pluvial waters would be able in time to remove the 

 coloring substances. How effective this pluvial leaching would be in 

 material so nearly impervious as consolidated underclays, the writer 

 cannot determine. On old outcrops of clays and clay shale at road- 

 sides, he has found little evidence of removal of iron and carbon. 

 There is usually a fixation of the iron while the bleaching, as a rule, 

 is insignificant — usually apparent rather than real and due to disinte- 

 gration or powdering. It may be that the English clays have been 

 exploited only along the outcrop but the case is dififerent in the 

 Appalachian basin. The tints are not confined to the outcrop. Clays 

 have been mined at several localities in Pennsylvania and Maryland 

 during 30 to 60 years, while in Ohio and West Virginia similar work 

 has been continuous for 60 to 80 years. Very many of the mines 

 work up the dip and are " bone dry " with thick cover, at times 

 hundreds of feet, through which no water passes. Pluvial leaching 

 has not existed there. The clay in these mines at a few feet from 

 the outcrop is like that obtained at 1,000 or 2,000 feet farther inside, 

 with pockets of varying tint and of varying composition — the latter 



