Page:The American Cyclopædia (1879) Volume IV.djvu/748

shale, &c., as are the coal beds of the carboniferous measures. Here also most of the beds of coal, if not all, are underlaid by soft white or gray clay, sometimes pure and used for pottery, like the clay of the carboniferous, sometimes dark-colored by heaped fragments of radicles of water plants. The shale and sandstone strata generally contain a profusion of remains of land plants, whose characters, related to this period, are far different from those of the carboniferous. They mostly represent palms with dicotyledonous species, oak, poplar, walnut, hickory, magnolia, cinnamon, &c. The flora of these tertiary measures of North America has afforded already many hundred species of fossil plants.—The combustible matter or the tertiary coal, often called lignite, is in its aspect scarcely distinguishable from true coal. At some localities, especially in the upper part of the measures, it is softer and more easily broken; but at others the matter is hard, compact, distinctly laminated in thin alternate layers of crystalline and more opaque matter, gives much heat by combustion, the best kinds in some localities, as at San Pete in Utah, even producing good coke, and when the beds are in proximity to upraised dikes of basalt or lava the coal is transformed into anthracite. Nothing then distinguishes this tertiary lignite from true coal but a less proportion of carbon in its compounds, and this can be recognized by chemical analysis only. The constituents are the same; only in the lignitic coal the decomposition is not as far advanced, as will be seen by comparing the analyses of the different combustible materials. In the upper tertiary, beds of lignite coal are still found, but they are less predominant, diminishing in number and thickness and in the extent of the area which they cover. The materials also in ascending gradually appear in a less advanced stage of decomposition.—In Europe lignite beds of the upper tertiary represent mere deposits of wood, especially trunks of trees transformed into a black soft substance, which preserve still the vegetable forms and also the texture of the wood, as well as if the trees had been recently cut. Later still, or rather nearer to our present epoch, the composition of the beds of lignite becomes undistinguishable from peat. To complete the similarity, old peat beds, as is especially the case in Holland, are found under thick strata of clay or sand, in two or more successive stages, or under thick gravel deposits of the drift of the quaternary. The material is mere soft peat, which by drying becomes hard as stone, and a combustible nearly as good as lignitic coal. Along the Ohio river and at various places in Ohio, Indiana, Illinois, &c., we find thick peat deposits in the clay of the drift. The bottom of the bed is clay; the combustible matter is true peat; the beds overlying it are generally hardened clay wherein leaves, nuts, acorns, &c., are found, most of these vegetable remains being already carbonized or hard and black as coal, as in the roof shale of the coal measures. In some localities trunks and branches of trees are imbedded in these clay formations; the woody substance of these vegetable remains is already softened and decomposed, and its place taken by the infiltration of clayey matter. They only need the hardening done by time to become fossilized trees, and henceforth, after the drift period, we have as representative of the carboniferous formations peat bogs of different ages: some very thick, overlaid by strata of humus or of sand or gravel; some more recent, covering the ruins of monuments of the human race, bridges, roadways, aqueducts, &c., or holding in their matter bones of extinct or nearly extinct races of animals, like those of the aurochs, with utensils, weapons, and ornaments, attesting the existence of races of men now unknown. But some deposits of peat are so recent that their beginning is positively remembered by old men, who have seen them grow to some thickness where as children they had seen only stagnant pools, or a forest, or a muddy swamp. All these peat deposits, like the beds of coal and of lignite, are found in connection with vegetable remains which are either dead and cover them entombed in the clay of their surface, or are still living and deposit and heap their woody matter upon them, thus constantly increasing their thickness. We must therefore acknowledge that from the beginning of what we call the geological formations, or from the base of the Silurian to our time, remains of plants are found in intimate connection with all the deposits classed under the denomination of combustible minerals: marine plants or seaweeds, with the strata containing oil or bitumen; land plants, with the deposits of coal in all its degrees of hardness and perfection, from the soft peat, which is merely unripe coal, to the hardest anthracite.—The origin of the coal, or at least its essential composition, is apparent from what has been said of its geological distribution. The first hypothesis thrown out on the subject was that the coal was a mere mineral or bituminous compound deposited and distributed in the series of the rocks like strata of other nature. As it is mostly in concordant stratification with them, it was therefore considered to have the same origin. But the formation of the strata of the earth is easily accounted for: the sandstone by erosion of primitive rocks, the removal, transportation, and deposition of the matter; the limestone by the submarine action of animal life, &c. As free bitumen does not exist in nature, deposits of bitumen in the rocks are an anomaly as long as the origin of bitumen cannot be positively indicated. Moreover, there is a great difference between bitumen, or rocks impregnated with bitumen, and coal beds. Hardened or oxidated bitumen has been found in a few instances filling crevices of rocks, as in Canada and in West Virginia; the matter is homogeneous, either semi-pellu-