THE FLORA OF THE COAL FORMATION. 4G5 



bark are separated by a fihn of clay or of mineral cliarcoal, occupying 

 the place of the wood. In this condition the bark of a large Sigillaria 

 gives only one or two lines in thickness of coal ; Stigmaria^ Lepido- 

 dendron, and Ulodendron give still less. In the shales these flattened 

 trunks are often so crushed together that it is difficult to separate them. 

 In the coal they are, so to speak, fused into a homogeneous mass. 



5. The phenomena of erect forests explain to some extent the 

 manner in which layers of compact coal and mineral charcoal may 

 result from the accumulation of tx'unks of trees in situ. In the 

 sections at the South .Toggins, the usual state of preservation of 

 erect Sigillarice is that of casts in sandstone, enclosed by a thin 

 layer of bark converted into compact, caking, bituminous coal, while 

 the remains of the woody matter may be found in the bottom of the 

 cast in the state of mineral charcoal. In other cases the bark has 

 fallen in, and all that remains to indicate the place of a tree is a little 

 pile of mineral charcoal, with strips of bark converted into compact 

 coal. Lastly, a series of such remains of stumps, with flattened bark 

 of prostrate trunks, may constitute a rudimentary bed of coal, many 

 of which exist in the Joggins section. In short, a single trunk of 

 Sigillaria in an erect forest presents an epitome of a coal-seam. Its 

 roots represent the Stigmaria undcrclay ; its bark the compact coal ; 

 its woody axis the mineral charcoal ; its fallen leaves, 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 show- 

 ing 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 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 earthy matter, pyrites, etc. 



6. The microscopic structure of the purer varieties of compact coal 

 accords with that of the bark of Sigillaria. The compact coals are 

 capable of affording very little true structure. Their cell-walls have 

 been pressed close together ; and pseudo-cellular structures have 

 arisen from molecular action and the segregation of bituminous 

 matter. Most of the structures which have been figured by micro- 

 scopists are of this last character, or at the utmost are cell-structures 

 masked by concretionary action, pressure, and decay. Hutton, how- 

 ever, appears to have ascertained a truly cellular tissue in this kind of 

 coal. Goeppert also has figured parenchymatous and perhaps bast- 

 tissues obtained from its incineration. By acting on it with nitric 



