454 Reviews and Notices of Books. 



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 

 tumps, with flattened bark of prostrate trunks, may constitute as 

 rudimentary bed of coal, many of which exists in the Jogg-ins 

 section. In short, a single trunk of Sigillaria in an erect forest 

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

 maria underclay ; 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 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 produc- 

 tion 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 un- 

 affected by causes which have filled those layers consisting of 

 herbaceous materials and decayed wood, with earthy matter, py- 

 rites, &c. 



" 6. The miscroscopic 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 seoreo'ation 

 of bituminous matter. Most of the structures which have been 

 figured by microscopists are of this last character, or at the ut- 

 most are cell-structures masked by concretionary action, pressure? 

 and decay. Hutton, however, appears to have ascertained a truly 

 cellular tissue in this kind of coal. Goeppert also has figured pa- 

 renchymatous and perhaps bast-tissues obtained from its incinera- 

 tion. By acting on it with nitric acid, I have fount! that the 

 structures remai^iing both in the lustrous compact coals and in 

 the bark of Sigillaria are parenchymatous cells and fibrous cells 

 probably bast-fibres. 



*' 7. I by no means desire to maintain that all portions of the 

 coal-seams not in the state of mineral charcoal consist of cortical 

 tissues. Quantities of herbaceous plants, leaves, &c , are also 

 present, especially in the coarser coals ; and some small seams 

 appear to consist entirely of such material, — for instance, of the 

 leaves of Cordaites or Poacites. I would also observe that, 

 though in the roof-shales and other associated beds it is usually 

 only the cortical layer of trees that appears as compact bituminous 



