Such occurrences as these have been supposed to offer arguments against the igneous origin of the trap rocks, but they prove nothing either way. The truth is, that they are never found in the crystalline rocks, nor indeed in the solid earthy ones of this family. They are always in some bed of clay, or tufa, or conglomerate, which, from their own unchanged conditions, could not have expe rienced much effect of heat. And as these tufas often contain rounded fragments, marking periods of motion in one place, or of transportation, it is easy to understand how fragments of wood should have• been entangled in them before the trap overwhelmed the whole. In those cases where the lignites are contained in fissures, the same explanation applies; for these fissures also contain tuface ous or loose matters. But even if such bituminized wood or coal should ever be found imbedded in the solid rocks, it would prove nothing against their igneous origin ; be :ause charcoal, under pressure, is not destructible in the highest heat, and because we can fuse wood in close vessels, and retain it there for ever, although with the loss of its vegetable texture.
There is no probability of our ever knowing what are the trees which have been the origin of any of the lignites ; at least with any degree of certainty, owing to the great :hanges which they have undergone. That of Cologne, whenever its texture is preserved, resembles fir. So does the specimen found in Mull that was just mentioned. Where the bituminizations have been complete, as in jet, there can be no chance of knowing. The gums or resins that have been found in some, as at Bovey, prove nothing, because they themselves have lost their character in a greater or less degree, and have acquired that of the bitu mens. They serve, however, like many other fossil re mains, to indicate changes in the inhabitants, if not in the climates of the globe. Thus the surturbrand of Iceland is the produce of far larger trees than that island can produce at present ; and thus trees must have once flourished in Melville island, where, at present, not a ligneous fibre exists.
These substances are among the few fossil remains which are of use. Like coal, those of Bovey, of Cologne, and of Iceland, are used for fuel, and the powdery variety of Cologne forms a beautiful brown paint. The uses of jet as an ornament are well known.
It is unnecessary to take any particular notice of fossil roots, as they are included in this division, as far as they are worthy of any notice. They were formerly distinguish ed by the term rhizomorpha ; and the fruits asd other parts of plants will fall more properly to be examined after we have dismissed the herbaceous vegetables, to which we shall now proceed.
On the Fossil Herbaceous Vegetables.
It has already been remarked, that the remains of plants were far less abundant, as well as less various, than those of animals ; nor is the reason difficult to assign, when we reflect on the manner in which the strata have originally been formed under the waters of the ocean. Marine
bodies were thus easily preserved, because they were speedily imbedded in the earths in which they had lived, and thus protected from further injury, while at the same time they underwent no motion, from transportation or otherwise, till they were secured in their solid coverings. Had marine plants been of a woody and imperishable na ture, the same would probably have happened to them, and we should thus have had vegetable remains also among the marine strata ; but being of a soft texture, and easily subject to decay after death, they have disappeared before they could be thus secured. And again, if we are inclined to ask why terrestrial vegetables were not brought down into the sea, together with the alluvia which are daily de posited there, the answer is easily found in their perish able nature. The smaller parts and leaves could not be transported far without utter destruction ; and if woods or trees should have escaped such injuries, yet their propen sity to float, often till they were rotted and destroyed, would prevent them from having been deposited among the sub marine materials, so as now to be found imbedded in the ordinary strata.
It will be seen, accordingly, that very few vegetable remains are found in the strata of marine origin, and that the great repository of them is that series in which coal is contained, and which, from this circumstance, as well as others, is proved to have been formed on the land in fresh water lakes and marshes. Wherever plants, or parts of them, arc really found in the marine strata, we must sup pose that they have been transported together with the alluvial matters, and so buried among them as to have been effectually secured from destruction till they became consolidated in the rock.
The lowest or oldest marine stratum in which vegetable remains occur, is the same argillaceous schist of the pri mary series that contains marine shells. This occurrence is, however, rare : it is not even quite certain that there may not have been errors in the observations, but the substances so found imbedded are said to be reeds and palms. It does not appear that any such have yet occur red in the two first of the secondary strata, namely, in the lowest red sandstone, and in the first, or mountain lime stone. It has, however, been said, that the fragments of ferns and reeds have been found in the bituminous marl slate, which is supposed to be a portion of the last named rock. We are rather inclined to think that this stratum belongs to the lias, a limestone far more recent ; the marl stratum, which lies below it, having been mistaken for the old red sandstone, as, in fact, it has been by Werner and his followers.