We will endeavour to establish here the true geological origin of coal, in order that no doubt may exist in the minds of our readers on a subject of such importance. In order to explain the presence of coal in the depths of the earth, there are only two possible hypotheses. This vegetable débris may either result from the burying of plants brought from afar and transported by river or maritime currents, forming immense rafts, which may have grounded in different places and been covered subsequently by sedimentary deposits; or the trees may have grown on the spot where they perished, and where they are now found. Let us examine each of these theories. [Illustration: Fig. 69.--Treuil coal-mine, at St. Etienne.] Can the coal-beds result from the transport by water, and burial underground, of immense rafts formed of the trunks of trees? The hypothesis has against it the enormous height which must be conceded to the raft, in order to form coal-seams as thick as some of those which are worked in our collieries. If we take into consideration the specific gravity of wood, and the amount of carbon it contains, we find that the coal-deposits can only be about seven-hundredths of the volume of the original wood and other vegetable materials from which they are formed. If we take into account, besides, the numerous voids necessarily arising from the loose packing of the materials forming the supposed raft, as compared with the compactness of coal, this may fairly be reduced to five-hundredths. A bed of coal, for instance, sixteen feet thick, would have required a raft 310 feet high for its formation. These accumulations of wood could never have arranged themselves with sufficient regularity to form those well-stratified coal-beds, maintaining a uniform thickness over many miles, and that are seen in most coal-fields to lie one above another in succession, separated by beds of sandstone or shale. And even admitting the possibility of a slow and gradual accumulation of vegetable débris, like that which reaches the mouth of a river, would not the plants in that case be buried in great quantities of mud and earth? Now, in most of our coal-beds the proportion of earthy matter does not exceed fifteen per cent. of the entire mass. If we bear in mind, finally, the remarkable parallelism existing in the stratification of the coal-formation, and the state of preservation in which the impressions of the most delicate vegetable forms are discovered, it will, we think, be proved to demonstration, that those coal-seams have been formed in perfect tranquillity. We are, then, forced to the conclusion that coal results from the mineralisation of plants which has taken place on the spot; that is to say, in the very place where the plants lived and died.