The bulk of evidence seems to favor this view, but there is another. Perhaps the coal-beds are not the sites of former swamps, but of estuaries and ocean shores where the plant material settled down, in still water, after a long drift down the ancient rivers from its place of origin. It is not impossible that both explanations are correct; some coal-beds having been formed in one way, and some in the other. With the progress of time the deposits of sand were compacted into sandstone, and the mud and clay into shale; while the layers of vegetation were solidified by pressure, some of their constituents were vaporized and expelled by heat, and the final product was coal. The coal-measures abound in fossil plants of species long ago extinct, and we also find the molds or casts of plants that have themselves disappeared, leaving only their impressions in the mud by which they were once enveloped. These records of ancient vegetation are mostly found in the rocks just above and below the coal-beds, and not in the coal itself. The plants of the Carboniferous Period, during which most but not all of the coal-beds were formed, bore a family likeness to certain kinds of plants that flourish today. Many of them were ferns, ranging in size from the smallest species up to great tree-ferns. Others resembled our modern horsetails or scouring-rushes, with their fluted and jointed stems, but these _calamites_, as the geologists call them, grew to the size of trees, sometimes eighty or ninety feet in height. Some plants of the coal age were like the modern cycads (intermediate in appearance between tree-ferns and palms); some were like the ginkgo, a tree with leaves like those of maidenhair fern, widely introduced into this country from China and Japan. One of the commonest and largest trees was the _lepidodendron_, closely resembling, except in its vastly greater size, the club-moss or ground pine which we know so well as a Christmas decoration. The animal life of the period, of which, also, abundant fossil remains are found, included mollusks, fishes, crustaceans, insects, spiders, thousand-legs, snails, reptiles and lizards. Some of the insects were a foot or more in length. Of cockroaches, alone, more than five hundred species have been found in the coal-measures. PREPARED BY THE EDITORIAL STAFF OF THE MENTOR ASSOCIATION ILLUSTRATION FOR THE MENTOR. VOL. 6, No. 6, SERIAL No. 154 COPYRIGHT, 1918. BY THE MENTOR ASSOCIATION, INC. [Illustration: COURTESY U. S. BUREAU OF MINES TIPPLE AT BITUMINOUS COAL MINE GARY, WEST VIRGINIA] _THE STORY OF COAL_ _The Coal Fields of the United States_ TWO