COAL: ITS STRUCTURE AND FORMATION. 



PART II. 



STRICTLY speaking, the subject of microscopical 

 structures in coal should include some account of the 

 sphaerosiderite nodules which occasionally occur in seams of 

 coal. It is from these nodules that Binney, Carruthers, 

 Williamson and others have obtained material for the ana- 

 tomical investigation of Coal-Measure plants ; the majority of 

 the invaluable treasures contained in Professor Williamson's 

 unique collection of Carboniferous plants have been prepared 

 from such nodules. According to Stur we should regard 

 these small nodular masses as patches of partially decomposed 

 plant substance, mineralised by carbonated waters, and thus 

 preserved as sample specimens of a peat-like accumulation 

 which was, for the most part, gradually converted into coal. 1 

 In the Reichsanstalt museum, in Vienna, there is an ex- 

 ceedingly interesting specimen of a block of coal containing 

 elliptical or spherical nodules of sphserosiderite, with some 

 pieces of crystalline igneous rocks embedded in a matrix 

 of coal. Stur considers that these mineralised patches of 

 peat lend considerable support to the growth-in-place theory 

 of coal formation ; but, as Grand' Eury has pointed out, it 

 is not difficult to understand the mineralisation of nests of 

 plant tissue by the action of petrifying solutions in a mass of 

 subaqueous vegetable ddbris. The occurrence of boulders 

 of different kinds of rock in coal beds is a subject of great 

 interest, but it is impossible to give any adequate summary 

 of the recorded facts in the present incomplete sketch. 2 

 Some geologists, with their firm belief in the autochthonous 

 formation of coal, suggested a meteoric origin for such 

 stones ; others adopted a more rational view, and regarded 

 them as erratic blocks which had been carried by floating 

 trees or ice, and finally dropped into a peaty deposit on the 

 floor of a lagoon. The occurrence of the so-called coal balls is 



1 See also Binney, p. 14. 2 For references, see Geikie and Stur. 



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