177

C.—3

On the way it lost certain portions of its vegetation, which, rising to the surface, carried with them spores giving birth to a fresh vegetation for the formation of a second bed of coal; or rather, after maceration, these vegetables partly redescended, in order to spread themselves out in the roof of the seam already formed. Moreover, when sinking through the muddy water of the lake during its descent, the Stigmaria of the first seam had drawn with them the solid particles in suspension, causing them to contribute to the formation of the bed of the seam. On the other hand, whilst the next seam was in preparation at the surface of the lake, the rocks at the roof of the first seam would be formed by the muddy or sandy deposits brought from the sides of the hills by the denuding action of the rains, as has been already stated. As it is not necessary to follow M. Breton through all the developments of his ingenuous theory, a few will now be pointed out. He assigns, for example, as the origin of the repetition of a seam by overlapping a local fault, a rent or tear in the descending seam, with superposition through lateral movement, and slight delay of one part on the other. He explains how nips are connected with swellings by detachment. During the downward movement a portion of the upper part of the seam was prolonged ; this portion, when detached, experienced a delay and a deviation, in consequence of which it was deposited, not in the corresponding cavity, which became filled with sediment, constituting a nip, but at the side, and gave a swell to the coal-seam. With respect to tree-trunks that are found upright in the rocks that separate the coal-seams, M. Breton supposes that, after the disorganization of their interior, and the replacement of this by the mud from the dusts which fell on the floating island, they penetrated through and fell vertically to the bottom of the lake, where they were surrounded with the sediments which were helping to form the sterile rocks between the lower seam already formed and the upper in process of formation. In support of this theory, M. Breton mentions the numerous floating islands described by ancient writers of the sixteenth, seventeenth, and eighteenth centuries in the marshes in the neighbourhood of Saint Omer, whose writings were rather historical than scientific. Nevertheless it seems to me that M. C. Grand-Bury has shown, as we shall see further on, that the materials constituting the coal have been the subject of a certain lateral transport, the effects of which would not have been produced by the vertical transport supposed by M. Breton. Besides, this theory gives no explanation of the separation, which is always so clean, and is habitually present in the Franco-Belgian basin, between the bed of coal and its roof. Neither does it explain sufficiently the separation there is between the seam and the wall, and this latter objection may also be urged against all the theories which suppose the formation of coal on place. Theory of M. G. Grand-Eury. M. C. Grand-Eury's study of the Carboniferous flora of the Loire Department, and of the centre of France, published in 1877 by this Institute, under the auspices of Admiral Brongniart, is the result of long and judicious research continued for more than ten years in underground workings, and the numerous plates and great extent of the work testify to the immense toil expended upon it. The eminent botanist, M. G. de Saporta, pronounced a eulogium upon it. Specially he called attention to the incontestable discoveries made by M. Grand-Eury on the reproductive organ of an important family of coal ferns, the Pecopteris, and on the gymnospermous nature and the affinities of the Cordaites —gigantic trees which played, especially in the upper coal, an important role almost unknown before the researches of M. C. Grand-Eury. In the publication in question this engineer also treats of the geogony of coal and the formation of the coal-basins. M. Grand-Eury has, above all, powers of observation of the highest order; he is an indefatigable worker, who, until now, has shown a sufficient reserve in making public the theoretical ideas with which his researches have inspired him. Since his last publication he has made numerous journeys into Eussia, Austria-Hungary, in Spain, &c, and has collected fresh documents, which he has not yet had leisure to make use of; but from 1887 he commenced to strike out the line which will probably conduct to the best theory possible of the formation of coal; and, if this object is attained, it will be due, above all, to the facts which he has brought to light, and to the continuance of his efforts in the same direction. We will indicate briefly the principal of these facts, and the conclusions that M. Grand-Eury has up till now deduced from them. The coal-beds exploited often present schistous intercalations, the sedimentary origin of which is without dispute. There are also impure coals formed of alternate layers of schist and coal. Prom this we may conclude that coal is a sedimentary rock, of the same class as the schist that accompanies it. Coal is formed, on the one hand, of thin parallel lammas, with a brilliant fracture, and which are generally composed of empty flattened bark and leaves ; and on the other, of a coaly substance, amorphous, more dull, the result of a very advanced decomposition of portions of vegetables, which yielded ulmic substances, still containing recognisable vegetable detritus, such as spores. The relative proportion of the organized and of the amorphous coal varies with the origin of the specimen. One or the other preponderates, but in the greater nuriiber of coals the last prevails. It even follows that the amorphous coal forms the greater part of the mass, which then is stratified only by the epidermic membranes, few leaves and scanty barks. This is what M. Grand-Eury designates " tegumentary coal." When the amorphous coal encloses abundant fragmentary detritus he names it " particle coal," and "vegetable pulp.' . All the vegetables of the coal-formation are invariably found in isolated and dispersed condition, the broken stems are often separated from the roots, and the appendicular organs are detached. Trees, more or less complete, such as are found in the actual marshes, or like those transported to the mouths of rivers, are not met with. With the exception of the fronds and spores of the ferns,