Page:Popular Science Monthly Volume 23.djvu/69

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HOW THE ANCIENT FORESTS BECAME COAL.

an essentially terrestrial formation, peculiar to the recently emerged land of the period. In Belgium and England they rest upon a marine deposit, which forms their floor, and which reappears in the course of the formation, alternating several times with the strata of land growth. We learn from this that the sea was retiring from these spots before the extension of the continental area, leaving a broader strip of land after each fitful inundation, and that the carboniferous vegetation was developed on the ground which the marine waters had just abandoned. This phenomenon acquires great force with its frequent recurrence and repetition in various places.

None of the carboniferous plants except the stigmarias, whose peculiarities we have noticed, appear to have been especially aquatic; but they could all endure the immediate neighborhood and occasional contact of water without being hurt by it, and could live and grow, even when partially inundated. They grew around the borders and on the slopes of the lagoons with which the shore was studded, the smaller ones thickly matted under the cover of the larger trees, in groups characterized by the predominance of single species, as is shown by the distribution of the fossils. The coal was deposited in the lacustrine beds at the center of these forest-covered depressions; and the extent of the deposits is measured by the area of the basins that were fitted to receive them. One condition was essential, without which no seam of combustible matter could have been formed. It was, that the water flowing over the ground should bring with it and leave in the bottom of the basin where the carboniferous matter was destined to accumulate, only the remains of plants, to the exclusion of every other form of sediment. This condition may have been more easily realized in the Carboniferous epoch than at any other time, because the flora was more abundant and its extension more favored by the climate. It is conceivable also that, after having been once established, it might have been liable to interruption at any time; for a slight oscillation of the ground, a change in the direction of the currents, the washing down of a bluff, or the removal of some impediment, may have been enough to furnish an opportunity for the introduction of sand, mud, or rock-dust, into the deposits. We may also affirm as essential that there should be no real affluent coming down to the place of deposit, or current of running water, for that would bring down mud, and leave in the bed some other sediment than one of coal. The flow of water must have been a gentle trickling over the soil, bathing it without washing it, but strong enough to carry along the vegetable matter which it finally deposited. Whenever the flow became more violent, the formation of coal was interrupted to give place to deposits of shale or sandstone, according to the character of the mineral elements brought down, or, if they were in relatively small proportion to the vegetable fragments, of schistose laminæ marked with impressions of plants.