106 



STEVENSON— FORMATION OF COAL BEDS. 



[April i8, 



Taylor"- compared the ash of good and bad coal with underclay, 

 bituminous shale and blue shale. The constituents are alike in all 

 though the relative proportions differ. The good coal with 1.36 of 

 ash and the poor coal with 16.9 of ash compared with the underclay 

 show 



Silica 



Alumina 



Sesquioxide of iron 



Lime 



Magnesia 



Potash 



Bischof thinks that the analyses show close relationship throughout 

 and that they indicate sedimentary origin for all the materials alike. 

 He says that the variation in composition of the earthy matters in 

 coal is not greater than in shales. McCreath has analyzed many of 

 the Pennsylvania fireclays and the results show great variation, the 

 silica from 47 to 66 and the alumina from 18 to 35 per cent. A 

 similar variation is found in the Pleistocene clays. The resemblance 

 between coal ash and the clays ought to be close in respect of con- 

 stituents, it matters not whether the coal is allochthonous or autoch- 

 thonous, but some of the differences offer abundant ground for 

 speculation. 



The large proportion of clay in coal ash is, for some, evidence 

 that the material is of extraneous origin, since clay is an extremely 

 unimportant constituent of plants. It is insignificant in the ash from 

 peat. Alills and Rowan"^ give analyses from 27 Irish localities 

 which show in the ash 0.129 to 10.705 of alumina, but 12 of them 

 have less than i per cent, and only 3 have more than 3 per cent. 

 It must be remembered, however, that the trees and the peat forming 

 plants of the recent period are not the same with those which 

 gave the coal. The most important plants during Coal Measures 

 time were lycopods and equiseta. Dana^^* cites analyses of some 



"' H. Taylor, cited by G. Bischof, " Elements of Chemical and Physical 

 Geology," London, 1854, pp. 268, 269. 



"' E. J. Mills and F. J. Rowan, "'Chemical Technology,"' Amer. Ed., Vol. 

 L, 1889, pp. 16-18. 



"* J. D. Dana, " Manual of Geology," 4th ed., 1895, pp. 74, 75, 663. 



