Fig. 38.—Stigmaria root, seen from above, showing its regular divisions.
From "Acadian Geology".
Fig. 39.—Portion of bark of Stigmaria, showing scars of attachment of rootlets.
Before considering other forms of Carboniferous vegetation, let us glance at the accumulation of coal, and the agency of the forests of Sigillariæ therein. Let us imagine, in the first instance, such trees as those represented in the figures, growing thickly together over vast swampy flats, with quantities of undergrowth of ferns and other plants beneath their shade, and accumulating from age to age in a moist soil and climate a vast thickness of vegetable mould and trunks of trees, and spores and spore-cases, and we have the conditions necessary for the growth of coal. Many years ago it was observed by Sir William Logan that in the coal-field of South Wales it was the rule with rare exceptions that, under every bed of coal, there is a bed of clay filled with roots of the Stigmaria, already referred to as the root of Sigillaria. This discovery has since been extended to all the coal-fields of Europe and America, and it is a perfectly conclusive fact as regards the origin of coal. Each of these “under-clays,” as they are called, must, in fact, have been a soil on which grew, in the first instance, Sigillariæ and other trees having stigmaria-roots. Thus, the growth of a forest of Sigillariæ was the first step toward the accumulation of a bed of coal. More than this, in some of the coarser and more impure coals, where there has been sufficient earthy matter to separate and preserve impressions of vegetable forms, we can see that the mass of the coal is made up of flattened Sigillariæ, mixed with vegetable débris of all kinds, including sometimes vast quantities of lepidodendroid spores, and the microscopic study of the coal gives similar results ([Fig. 40]). Further, on the surfaces of many coals, and penetrating the shales or sandstones which form their roofs, we find erect stumps of sigillaria and other trees, showing that the accumulation of the coal terminated as it had begun, by a forest-growth. I introduce here a section of a few of the numerous beds of coal exposed in the cliffs of the South Joggins, in Nova Scotia, in illustration of these facts. We can thus see how in the slowly subsiding areas of the coal-swamps successive beds of coal were accumulated, alternating with beds of sandstone and shale (Figs. [41], [42]). For other details of this kind I must refer to papers mentioned in the sequel.
Fig. 40.—Vegetable tissues from coal. a, Sigillaria and Cordaites. Calamodendron.