THE GROWTH OF COAL 237 



ribs and scars of Sigillaria or other coal-formation trees on 

 its surface. In other words, the layers of fine coal are usually 

 flattened trunks and branches of trees, or perhaps rather of the 

 imperishable and impermeable bark of such trees, the wood 

 having perished. A few very thin layers of shining coal we may 

 also find to consist of the large-ribbed leaves of the plant known 

 as Cordaites. This kind of coaly matter then usually represents 

 trunks of trees which in a prostrate and flattened state may 

 constitute more than half of the bulk of ordinary coal-formation 

 coal. Under the microscope this variety of coal shows little 

 structure, and this usually the thickened cells of cortical tissue. 

 Intervening between these layers we perceive laminae, more or 

 less thick and continuous, of what we may call dull coal, black 

 but not shining ; resembling, in fact, the appearance of cannel 

 coal. If we split the coal along one side of these layers, and 

 examine it in a strong light, we may see shreds of leaf stalks 

 and occasionally even of fern leaves, or skeletons of these, show- 

 ing the veins, and many flattened disc-like bodies, spore cases 

 and macrospores, shed by the plants which make up the coal. 

 These layers represent what may be called compressed 

 vegetable mould or muck, and this is by no means a small 

 constituent of many coals. This portion of the coal is the 

 most curious and interesting in microscopic slices, showing a 

 great variety of tissues and many spores and spore cases. 

 Lastly, we find on the surface of the coal, when split parallel to 

 the bedding, a quantity of soft shining fibrous material, known 

 as mineral charcoal or mother coal, which in some varieties of 

 the mineral is very abundant, in others much more rare. This 

 is usually too soft and incoherent to be polished in thin slices 

 for the microscope ; but if boiled for a length of time in nitric 

 acid, so as to separate all the mineral matter contained in it, 

 the fibres sometimes become beautifully translucent and reveal 

 the tissues of the wood of various kinds of Carboniferous trees, 

 more especially of Calamites, Cordaites and Sigillariae. Fibres- 

 s. E. 17 



