themselves; but existing in the heart of the most solid rocks, often nicely crystallized, and completely inclosed, on all sides, without the most minute vacuity. The pyrites must have been present, therefore, when the strata were consolidated, and it is inconceivable, if their consolidation was brought about in the wet way, that a substance should be so generally found in them, the very existence of which is incompatible with humidity. This argument for the igneous origin of the strata is applicable to them all, but especially to those of coal, as abounding with pyrites more than any other. 29. The difficulty that here naturally present itself, viz. how vegetable matter, such as coal is supposed to have been, could be exposed to the action of intense heat, without being deprived of its inflammable part, is obviated by the principle formerly explained concerning the effects of compression. The weight incumbent on the strata of coal, when they were exposed to the intense heat of the mineral regions, may have been such as to retain the oily and bituminous, as well as sulphureous parts, though the whole was reduced almost to fusion; and thus, on cooling, the sulphur uniting with iron might crystallize, and assume the form of pyrites. 30. The compression, however, has not in every instance preserved the bituminous, in union with the carbonic part of coal; and hence a mark of the operation of fire quite peculiar to this fossil, and found in those infusible kinds of it which contain no bitumen, and burn without flame. These resemble, some of them very precisely, and all them in a great degree, the products obtained by the distillation of the common bituminous coal that is, they consist of charcoal, united to an earthy basis in different proportions. It is natural therefore to conclude, that this substance was prepared in the mineral regions by the action of heat, which, in some instances, has driven off the inflammable part of the coal. That the heat should, in some cases, have done so, is not inconsistent with the general effect attributed to compression. The conditions necessary for retaining the more volatile parts, may not have been present every where in the same degree, so that the latter, though they could not escape, may have been forced from one part of a stratum, or body of strata, to another.