We have remarked above that the portions of Lepidodendron analyzed belonged to that part of the bark that was considerably thickened and lignefied.  So too the portion of the Megaphyton that was submitted to distillation was the external part of the hard bark, formed of hypodermic fibers and traversed by small roots.  The Psaronius, on the contrary, was represented by a mixture of roots and of parenchymatous tissue in which they descend along the trunk.

It results from these remarks that we may admit that those parts of the vegetable that are ordinarily hard, compact, and profoundly lignefied furnish a compact coke and relatively less volatile matter, while the tissues that are usually not much lignefied, or are parenchymatous, give a bubbly, porous coke and a larger quantity of gas.  The influence of the varied mode of grouping of the elements in the primitive tissues is again found, then, even after carbonization, and is shown by the notable differences in the quantities and physical properties of the products of distillation.

The elementary chemical composition, which is perceptibly the same in the specimens isolated in the sandstones and in those taken from the great deposit, demonstrates that the difference in composition of the environment serving as gangue did not have a great influence upon the definitive state of the coal, a conclusion that we had already reached upon examining the structure and properties of the coal pebbles.

We may get an idea of the nearly similar composition of the coal produced by very different plants or parts thereof, in remarking that as the cells, fibers, and vessels are formed of cellulose, and some of them isomeric, the difference in composition is especially connected with the contents of the cells, canals, etc., such as protoplasm, oils, resins, gums, sugars, and various acids, various incrustations, etc.  After the prolonged action of water that was more or less mineralized and of multiple organisms, matters that were soluble, or that were rendered so by maceration, were removed, and the organic skeletons of the different plants were brought to a nearly similar centesimal composition representing the carbonized derivatives of the cellulose and its isomers.  The vegetable debris thus transformed, but still resistant and elastic, were the ones that were petrified in the mineral waters or covered with sand and clay.  Under the influence of gradual pressure, and of a desiccation brought about by it, and by a rising of the ground, the walls of the organic elements came into contact, and the physical properties that we now see gradually made their appearance.

The waters derived from a prolonged steeping of vegetables, and charged with all the soluble principles extracted therefrom, have, after their sojourn in a proper medium, deposited the carbonized residua that have themselves become soluble, and have there formed masses of combustibles of a different composition from that resulting from the skeletons of plants, such as cannel coal, pitch coal, boghead, etc.