and sandstones, so as to show that there was nothing universal in the phenomenon, and that it was liable to interruption by physical changes. It is also easy to conceive that the formation of coal could not have gone on unless the vegetation was adapted to the process 3 and the conditions of the climate were suitable. The coal-plants could never have grown and flourished as they did in the present climates of the North; and our hard-wood trees, with their firm foundations in the ground, and their slow, periodical growth, could never by decaying in the open air have produced the peculiar and rich combinations we find in the coal-beds. The nature and bearing of these three concurrent factors have been carefully studied out by M. L. Grand' Eury, who has for that purpose spent many years in personal inspection of various mines and their surroundings, and has presented, in his "Mémoire sur la formation de la houille" ("Memoir on the Formation of Coal," Paris, 1882), a complete theory on the subject, including a review of the details of the process as taught by his observations of the phenomena.
The plants of the coal-measures, so far as their nature has been revealed to us by their remains, were great ferns, gigantic lycopodiums, called by the geologists lepidodendrons, and calamites and asterophyllites, allied to existing Equisetaceæ; all referable to the class of cryptogams. Besides these was another group, the character of which was long problematical, composed of the sigillarias and stigmarias. It now appears to be established that the stigmarias were a kind of rhizoma which had the faculty of persisting for a long time under the mud unchanged, growing and multiplying by stolons, but incapable in that condition of producing sexual organs; while under favorable circumstances they formed enormous buds whence shot up to the height of a hundred or a hundred and fifty feet the huge leaf-clad stems whose fossils, marked with the beautiful scars representing the leaf attachments, have been called sigillarias. Gymnosperms were also quite plentiful, of one class of which, the cordaites, M. Grand' Eury has made some happy restorations. He has found their leaves and seeds in considerable abundance at Saint-Étienne, and he has observed at the same place visible traces of their carbonized trunks, still standing erect and traversing the sandstone strata of some of the quarries.
A peculiar feature of these plants was the extraordinary predominance of the cellular or succulent tissues in them, and the corresponding rarity of the hard or fibro-ligneous parts, which appear reduced to insignificant cylinders. It was certainly not the destiny of these parts to increase with time, after the manner of the wood of our trees; and, in examining the mature stems of the ancient plants, we never find any more than an extremely thin ring of real wood. The rest is all pith, and even the bark, except on the outside, frequently presents an open or spongy texture. Such structure is similar to that of the aquatic plants of the present time, which can not exist at all in the air, and wither as soon as they are taken out of the water. An atmos-
