All parts of these galleries, indeed, were riddled as it were with these trap veins running here and there in the coal, and also in the sandstone, altering the coal to a greater or less extent according to the bulk of the trap, but producing very little apparent alteration in the sandstone.
The men in this colliery call this white rock the "door-case rock," because in one of the gate-roads formerly driven in the part of the coal which is generally devoid of trap, an upright vein of it crossed the gate-road like a wall, and appeared when cut through like a white door-case in the black coal.
From the fine tortuous and branching veins which this rock forms, it seems hardly possible to suppose that at the time of its injection it was not in a fluid state, and almost perfectly liquid. The distances to which these narrow veins run seem also to make it necessary to suppose that at the time of injection it had a temperature not merely just sufficient to melt it, but a much higher one, sufficiently high to allow of the loss of a considerable quantity of heat, and yet for the matter to remain still molten in its passage to very considerable distances from the volcanic focus. Doubtless the injection was a rapid one, but still some heat must be lost in the passage of these comparatively thin sheets of molten matter over such very considerable areas.[1]
Now, if masses, however comparatively small, having such an intense heat as above supposed, come in contact with a substance so easily affected as coal, we should at first expect it to be almost entirely consumed, or at all events altered to a much greater extent than it appears to have been. Our ideas, however, are derived from witnessing the effects of heat at the surface, and in contact with the atmosphere. Might not these ideas lead us astray, if we reasoned from them as to what would take place at some depth in the earth, unless we made allowance for the total or nearly total absence of air, and the effect of the covering of rock, which would greatly obstruct the dissipation of the constituent gases of the substance acted on, rendering that dissipation very difficult and therefore very small?
Is it altogether impossible that the nature of the alteration was rather the extraction of a certain portion of the hydrogen and oxygen of the coal, together with some of its carbon, and the combination of those substances with the oxides previously existing 1p the igneous matter, rather than the expulsion of those gases into the rocks above or below the coal? In this way the greatest amount of metamorphosis would be produced in the substance of the igneous rock, some part of the matter of the coal being actually absorbed into it and used up by it, and its constituents combined with its own as it passed along. The blackened margin of the coal
- ↑ M. Delesse, in his "Etudes de Metamorphism," supposes in these cases that the intrusive rock had not such an intense temperature, but that it was mixed with water into a state resembling mortar. It does not appear to me, however, possible for a mere paste to be injected into fine crevices to such great distances as we find in the above and other cases.
