THE CARBONIFEROUS FLORA. I45 



cases would be most suitable material for the production 

 of bituminous coal. But this suitableness they share with 

 the epidermal tissue of the scales of strobiles, and of the 

 stems and leaves of ferns and lycopods, and, above all, 

 with the thick, corky envelope of the stems of Sigillarim 

 and similar trees, which, as I have elsewhere shown,* 

 from its condition in the prostrate and erect trunks con- 

 tained in the beds associated with coal, must have been 

 highly carbonaceous and extremely enduring and im- 

 permeable to water. In short, if, instead of " spore-cases," 

 we read "epidermal tissues in general, including spore- 

 cases," all that has been affirmed regarding the latter will 

 be strictly and literally true, and in accordance with the 

 chemical composition, microscopical characters, and mode 

 of occurrence of coal. It will also be in accordance with 

 the following statement, from my paper on the "Struct- 

 ures in Coal," published in 1859 : 



" A single trunk of Sigillaria in an erect forest pre- 

 sents an epitome of a coal-seam. Its roots represent the 

 Stigmaria underclay ; its bark the compact coal ; its 

 woody axis the mineral charcoal ; its fallen leaves (and 

 fruits), with remains of herbaceous plants growing in its 

 shade, mixed with a little earthy matter, the layers of 

 coarse coal. The condition of the durable outer bark of 

 erect trees concurs with the chemical theory of coal, in 

 showing the especial suitableness of this kind of tissue for 

 the production of the purer compact coals. It is also 

 probable that the comparative impermeability of the bark 

 to mineral infiltration is of importance in this respect, 

 enabling this material to remain unaffected by causes 

 which have filled those layers, consisting of herbaceous 

 materials and decayed wood, with pyrites and other min- 

 eral substances." 



* " Vegetable Structures in Coal," " Journal of Geological Society," 

 XT,, 626. " Conditions of Accumulation of Coal," Und., xxii., 95. " Aca- 

 dian Geology," 197, 464. 



