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



beds is not local; they are mined in all parts of their areas and yield 

 scores of millions of tons each year; the analyses are of commercial 

 samples so that they show more ash than the coal itself would show, 

 apart from the thin partings of mud due to overflows. These coals 

 have less inorganic matter than the plant substance should have 

 yielded, which shows that, where accumulation proceeded in a 

 normal way, the product is likely to contain diminished ash. In 

 advancing change by metamorphosis or otherwise the ash is reduced, 

 as appears from analyses of the New Mexico coal and of coals from 

 the anthracite fields of Pennsylvania. 



It is wholly probable that not a little of the original inorganic 

 content was removed in solution. Maceration takes much from 

 flax and Fayol ascertained that the same efifect is produced on hemp. 

 Wood floated down the Rhine loses much during the journey. Be- 

 sides this, the organic acids form slightly soluble salts with several 

 bases, which would be removed by leaching. Evidently some areas 

 in southeastern Kentucky, where a coal bed shows less than i per 

 cent, ash in commercial samples, must have been in an exceptionally 

 favorable position, wdiere the accumulating coal was protected from 

 flooding by muddy water but exposed to leaching. 



The Roof. 



The normal roof of a coal bed is shale, often resembling that 

 of the mur in composition but differing in structure.. Roof shale is 

 more or less laminated but ordinarily there is no trace of lamination 

 in the underclay. In what may be termed normal conditions, the 

 passage from coal to roof is gradual, there being a faux-toit, in 

 which foreign matters increase gradually until at the top all traces 

 of coal have disappeared. This may be a bone or a bony coal, with 

 external appearance of cannel, or it may be a coarse worthless coal, 

 made up of alternating layers of bright coal and black shale loaded 

 with leaves or flattened stems. It may be only a few inches thick or 

 it may continue, as in the Pittsburgh bed, through 3 to 16 feet of 

 measures. Sometimes, the passage is abrupt, as seen at the partings 

 or, so to say, the subordinate roofs of a coal bed, which, as has been 



