Page:The American Cyclopædia (1879) Volume IV.djvu/749

Rh like dark-colored glass (the albertite), or a dull black compound comparable by its appearance to cannel coal (the grahamite), but filling holes, without concordance of stratification with the rocks wherein it is deposited, and proved by chemical analysis to be mineral oil oxidated and thus solidified by long exposure to atmospheric influence. Cases of this kind explain nothing concerning the origin and nature of coal, and therefore the first hypothesis was soon set aside.—That coal is composed of woody matter or of vegetable remains is easily recognized by ocular examination. In carefully inspecting a piece of coal the observer will in most cases see it formed of thin parallel layers of semi-transparent or crystalline matter, alternating with more opaque, earthy ones of the same thickness. These layers, about the tenth of an inch thick, are distinct enough to be counted like the rings indicating annual growth upon the horizontal section of a tree; and this even upon anthracite as well as bituminous coal. In splitting a piece of coal in the plane of stratification or of the layers, the exposed surface generally bears a pulverulent matter resembling charcoal; and this under the microscope is seen to be composed merely of vegetable fibres. Often the original form of the plants from which these woody fibres are derived (bark of lepidodendron, sigillaria, calamites, leaves and stems of ferns, &c.) is visible to the naked eye. In some cases a piece of coal a few inches square is seen with its faces covered with branches and leaves of ferns perfectly distinct in their outline and nervation; and in that way as many as five species have been identified upon the same surface, from their carbonized skeletons. In other cases, as in cannel coal, the fibrous texture of the matter cannot be recognized at first; but if thin layers of such coal are exposed to the action of a strong acid, the black bituminous substance is dissolved and the fibres are exposed whitened and distinct. Interesting researches have thus positively established the fact that coal is composed of vegetable debris. This conclusion agrees with the facts recorded above in regard to the distribution of the coal strata, which in the geological formations of all the epochs have been seen always in connection with vegetable remains.—But the essential fact, the origin of coal from vegetables, being admitted, the mode of nature's proceeding is not yet explained; and the question is still, whence have been derived the woody materials for the composition of the coal, and how have they been brought together and heaped in such immense masses of combustible as are represented by coal beds 10 to 20 ft. thick or more, extending over areas of many thousand square miles? Only two hypotheses are worth considering: that of the transportation and heaping of woody materials by water, and that of the growth of the materials upon the same surface now occupied by the coal beds. The first theory presupposes

that forests growing upon slopes along lakes or seashores had been torn down by whirlwinds, and that the trees had been then carried by floods to the bottom of the lakes or to the sea, and there entombed and transformed into coal. This hypothesis could scarcely account for the formation of coal beds of very small extent. In the transportation of trees or vegetable remains by water, a large proportion of sand, mud, &c., would of course have been swept down with the forests, and mixed in the deposits with the wood, rendering the matter very impure. Moreover, the coal deposits are generally in flat basins of wide extent, and an equal distribution of the trees in horizontal layers is an impossibility by transportation of this kind. Another difficulty in the amount of woody matter which enters into the composition of a coal bed is not generally known, and is far above every hypothetical calculation. Considering areas of the same extent, a coal bed 6 ft. thick is equivalent to all the wood which could be produced by a forest in 2,400 years, and this supposing that, as is done in the government forests in France, all the wood should be cut in its prime and the cultivation of the forests cared for in order to force the productiveness to its highest degree. Some beds of the carboniferous measures of North America, the Pittsburgh coal, and the big vein, with an average thickness of 6 ft. at least, cover an area of more than 20,000 sq. m. Who would dare to suppose the production of such a mass of vegetation along a river, its carriage to the sea for 2,400 years, and its deposition upon a continuous surface in horizontal strata of an even thickness? Moreover, the lamination of the coal, its horizontal and continuous extent over wide areas, the formation of the shale above the coal (shale generally mixed with an abundance of vegetable remains, some of them very large), the presence of standing trees or standing petrified forests either in the clay beds under the coal, or even imbedded in the coal beds, or rooting in the clay beds above—all these facts, seen in connection with the coal formation and a number of others, are inexplicable by the theory of transportation. We have therefore to come to the second hypothesis, and to see whether coal beds proceed, like the peat beds of our time, from the growth of vegetables the debris of which have been yearly and successively heaped in place and then transformed into coal. This is called the peat-bog theory.—The formation of peat is generally little known or understood. Few works have been published on the subject, and as the bogs are generally of difficult and even of dangerous access, they are rarely examined carefully enough to obtain full evidence as to the details of their formation. And furthermore, this study demands a knowledge of botany and chemistry rarely attainable by the student before the years of his strength for field explorations are passed. Peat is formed in shallow water or in bogs, by the growth of