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THE OEIGIN AND THE MODE OP THE FOEMATION OP COAL. The origin and mode of the formation of our coal-deposits is a subject which has been discussed by many scientists, and different ideas and conclusions arrived at as to the formation of coal. All agree that coal is of a vegetable origin, but the question is, What was the peculiar vegetation, and why do we find the coal in such thick seams ? On the sth November and the 10th December last an interesting paper was read by Adolph Firket, Professor of the University of Liege, at the meetings of the Association of Engineers, of which he is president, from which the following extracts are taken. He states, in reference to the origin of coal, that the question is certainly not new, and for a long time it has even ceased to be agitated ; when it was drawn from its retirement and once again restored to the order of the day by very interesting works, of which the earliest go back fifty years, and the series would appear far from being exhausted. These works have imparted to the subject a fresh reality. Generalities and Ancient Theories. That coal is of vegetable origin is indisputable, and, indeed, is hardly ever contested. It is generally conceded that it is the result of vegetable accumulation, or of the accumulation of vegetable debris, terrestrial, aerial, or marshy. Nevertheless it is possible that in certain exceptional cases marine Algae have contributed to its formation ; besides, in certain bog-heads and cannel-coals, the origin of which is hardly to be distinguished from that of coal, there exists, by the side of the vegetable matter which forms the principal mass, accessory portions of animal origin. There is an old hypothesis according to which coal should have been emitted by volcanoes in the state of viscous mineral bitumen. Although this theory has been reproduced quite recently by M. Judycki, who has supposed that this bitumen is liberated in a semi-fluid condition by the faults, in order to be next deposited in the existing depressions, while the schist coals proceed from volcanic sediment, yet this hypothesis is so out of harmony with the results of an attentive study of the coal-seams and of the rocks in which they are imbedded, that it is not feasible. The hypothesis of a physician, who conjectured that during the coal period there fell frequently from the atmosphere showers of hydric-carburets, gaining the low grounds, and there forming coal, still less merits discussion. Nevertheless, if the savants and the engineers who are seriously engaged in the geogony of coal are unanimous in ascribing to it an essentially vegetable origin, they are far from being of one mind as regards the mode of accumulation of the vegetables and of the debris of vegetables of which it is formed. The more exclusive either suppose that the coal was formed on place, or they there see only one depot of transport; but there are other authors who, as they consider that either one or other of these two modes of formation preponderate, accord to the other a slight influence. According to M. G. de Saporta it is in a memoir of the botanist, Antoine de Jussieu, relative to the coaly-vegetable impressions of Saint Chaumont, in the neighbourhood of Lyons, presented in 1718 to the Paris Academy of Sciences, that we find the first notions tending to establish the vegetable origin of coal. In these impressions De Jussieu especially noticed ferns differing from those with which he was acquainted, and which he supposed either no longer existed or came from distant countries, the flora of which were then unknown. He noticed that they were laid down flatly between schistous leaves, as between the pages of a herbal, and he supposes their transport by waters from their place of origin. The existence of fossil vegetables at the roof and at the side of the coal-seams next sufficed for a long time to cause the majority of naturalists to admit that this combustible was the result of vegetable accumulation, for the discovery of evident traces of vegetable organization in the coal is relatively recent. Coal lends itself but little in general to microscopic examination by transparency in thin leaves. Not to speak of the difficulty with a substance so friable of obtaining good preparations it would seem that usually the walls of the vegetable cells and the vessels disappeared at the time of its transformation into coal, only to leave an amorphous mass. However, either by this mode of observation, or by microscopical examination of the superfices by means of reflected light, often after treatment of the substance with concentrated nitric acid, or with a mixture of this acid with chlorate of potassium, Witham, J. W. Dawson, Bailey, and J. Quekett, in England and America; Go'ppert, C. W. Gumbel, and P. E. Eeinsch, in Germany; B. Eenault and C. Eug. Bertrand, in France ; and also other competent observers, by the employment of magnifying powers reaching, in the greater number of cases, to between 100 and 300 diameters, have found evidence of the existence of spores, of -portions of vessels, or even of more complete vegetable organisms in various specimens of bog-head, cannel-coal, coal, and anthracite. As far as coal is concerned, the evidence is especially thin sections in the external coalified portions of trunks of Calamite, Calamodendron, Lepidodendron, &c, of the coal-beds, trunks usually filled internally with a gritty or psamitic matter, in which we see a well-preserved organized structure. The vegetable origin of this species of coalified bark being certainly beyond question, its physical and chemical resemblance to coal, properly so called, commits us, moreover, to extend to coal itself the inference of vegetable organization more complete than the microscope has revealed. By aid of a powerful magnifying-glass, or even with the naked eye, several observers, amongst whom MM. C. Grand-Eury and H. Fayol, have found evident traces of vegetable structure at the surface of coal-lamellse. There are in my possession two different varieties, which, owing to an extremely thin schistous interposition, vegetable structure of the coal is very evident. One is an impure coal, taken from a small seam of the lower part of the Liege basin. It consists of thin lamellae of coal, the thickness of which is not greater than a millimetre, separated by thin schistous leaves, having preserved the imprint of the vegetables which contributed to the formation of these lamellse, among which a trunk of the Lepidodeudron can be easily recognised.