Page:The American Cyclopædia (1879) Volume IV.djvu/752

736 and distribution of their materials can be studied upon the exposed face of the beds, these compounds are seen constantly and extremely variable: heaped remains of woods, of mosses, sometimes whole forests prostrated and imbedded. But in these beds the alternation of the annual layers is always easily recognized. Near the top their thickness is generally more than an inch; near the base they become by maceration and compression reduced to one sixth of an inch or less. In the oldest peat bogs, the lower layers are formed of a soft pellucid matter resembling a black glue, alternating with a more opaque and fibrous material, thus exactly corresponding in their appearance to the pellucid and opaque laminæ of the coal. Hardness or induration of this old peat seems to the eye the only process needed to transform it into coal. The submerged peat, that of the lakes or of the seashores, frequently and extensively formed in Holland, North Germany, and elsewhere, is not laminated; the matter is generally compact, homogeneous in aspect, and nearly reduced to paste, at least in its more advanced stage of decomposition. The difference is caused, as remarked already, by the semi-aquatic nature of the plants which enter into its composition, and which, being less fibrous than those growing upon emerged bogs, are more easily and equally transformed under water. This process explains the difference between the laminated bituminous coal and the compact homogeneous cannel. The immense trunks mixed in the beds of laminated coal evidently prove that some of the beds were formed above water. On the contrary, cannel coal, whose compounds, so far as they can be recognized as vegetable remains, are essentially the floating trunks and branches of stigmaria, indicates as evidently that in this case the coal was formed of plants floating or submerged, thus presenting the two essential differences which are recognized in the peat beds of our time. Local variations in the phenomena attending the formation of immersed peat are quite as frequent and remarkable as in the formation of the emerged peat bogs. It would require a volume to describe them in detail. Near Kiöogge in Denmark, along the seashore, extensive peat deposits now covered by a layer of humus forming prairies are composed merely of birch trees, whose bark is separated from the decomposed woody matter; this is a semi-fluid brown paste at the bottom, 4 to 6 ft. thick, and the bark at the surface forms a layer of hollow cylinders or compressed sheets without any woody matter between them. The exploitation of these deposits is made to get the woody paste, which is taken in buckets from under the layer of bark, spread upon beds of straw, and when somewhat hardened by the percolation of water is kneaded, flattened, and pressed with shovels, and then cut and dried, thus forming a very valuable fuel. The bark is left out of

the trenches as of little value. We have in this case the illustration of a peculiar formation of some beds of coal whose top is overlaid by a stratum mostly composed of pieces of bark of lepidodendron, sigillaria, calamites, &c., pressed upon one another without even laminæ of coal between the layers. At Trevorton in Pennsylvania, a bed of semi-anthracite thrown up to the perpendicular is roofed by such a layer of petrified bark, and exposed like a wall of mosaic work, diversified by the impressions which mark the surface of the bark of most of the species of trees of the carboniferous period.—It has been objected to this theory of the production of coal, that the peat beds, however thick they may be, could not account for the immense mass of combustible minerals or of woody matter contained in a coal bed even of moderate thickness. This is a great mistake, for indeed nothing but the growth of the materials upon the place where they have been buried and transformed into coal can satisfactorily account for the immense proportion of woody matter in the deposits. The comparative caloric power of wood, peat, and coal has been found to be on an average represented by the proportion of 30 for wood, 37 to 40 for peat, according to its age, and 60 for bituminous coal. We have seen above what is the average production of wood under the best regulated cultivation of Europe. The growth of peat, as recognized from repeated observations, also made in Europe, averages one inch a year at the surface. By compression and decomposition the matter is reduced toward the base of the banks to one sixth or even one eighth of the original thickness. Comparing this production with that of the wood of a forest upon an equal area and in the same period of time, we see from the same records quoted above that the peat yields in cubic feet, after desiccation, twice as much combustible matter as the forest. Peat bogs are now, especially in wet countries, as in Ireland, Sweden, and Russia, not only of wide extent but of great thickness. In Switzerland and Germany the average thickness of peat beds is 8 to 12 ft.; but in the same countries the peat attains in some localities a thickness of 30 ft. in continuous and extensive beds. In Ireland, Denmark, and other northern countries, there are beds of combustibles formed of peat alternating with prostrated forests, from 60 to 80 ft. thick or more. Now, a bed of peat 30 ft. thick, comparing its matter with coal according to the proportion of 40 to 60 for caloric produced, and deducting one half for drying the matter and rendering it fit for fuel, would represent an amount of woody matter equal to that of a bed of coal 10 ft. thick. It is therefore conceivable that at the carboniferous period, when the atmosphere was saturated with vapors and carbonic acid gas, the essential elements which feed the plants and supply woody tissue, the luxuriance of the vegetation must have been far above that of our time. The heaping of