These are the only examples that have yet been pro duced from the tribe of Lichens; and we need scarcely add that all the rest belong to the smaller c•yptogamous plants; the only other ones indeed known, except the confervaz, being mosses. Of these, for similar reasons, the general characters alone can be ascertained, and in the paper above mentioned, figures of two are given. These resemble, in their general aspect, either the genus Hypnum or Junger mannia. If there were any doubt of their real nature, it would be removed, as far as the highest botanical autho rity is competent to do so, by saying that Mr. Brown was satisfied that the specimens belonged to one or other of these ; although he could not determine which. Number less other figures are given in the same paper, of fibrous substances and of fragments which appear to be detached leaves, together with some that resemble the fructifica tions of mosses, but of which the real nature is doubtful. But as no other kinds have been distinctly ascertained, we need dwell no longer on this part of the subject.
It is a natural question to ask whether the plants thus preserved are specimens of existing species, or whether, like those found entangled in the secondary strata, they are the remains of a former set of organized beings. This is a question, however, that cannot be answered by anato mical investigations, for the reasons already assigned; but Daubenton's observation would decide in favour of the former supposition. It is probable also, from general con siderations, that this is the fact, because the deposition of the chalcedonies by which they are entangled is a matter of daily occurrence. Nor have we any reason to doubt that they occur now in situ, from the appearances of some of the Siberian specimens that have been produced.
With respect to the manner in which this operation must have been performed, there seems to be no manner of difficulty; and in discovering it we even receive assist ance from many of the chalcedonies which contain chlorite instead of plants, as the process is very nearly the same in both. In these, which occur very often" in Iceland, the stalactitic chalcedony is produced by filtration through trap rocks into cavities and crevices ; and it is indeed in this manner that all the onyx nodules of the trap rocks, whether stalactitical or not, are formed. In such cases either the zones are deposited in succession all round the interior of the cavity, so as to produce the common coated agates, or else the specimen consists of a stalactite, or of a stalagmite, or of both together. In this latter case the stalactite is first formed, at least to a certain length ; and then, as the stalagmite rises upwards from the bottom, it entangles the drop, so as to produce that complicated ap pearance of horizontal tables of chalcedonic matter pene trated by stalactitic cones. Now in these cases the chlo rite fibres are covered by the flinty matter ; often, at the same time, forming, as it were, the basis or central fibre of the stalactite.
It is exactly by the same operation that the plant is en tangled ; and it is equally plain that it may be imbedded in either. If it hangs from the upper part of the cavity, it serves as a conductor for the flinty matter, which thus becomes covered by it ; while, if it grows on the bottom of the cavity, it is imbedded in the stalagmite. It is plain
that, for such an operation, the cavity must be an open one; while in the case of chlorite, the concretion may be formed, as we know it to be, in close ones. That this is truly the fact, we have ascertained by means of a large mass of chalcedony formed in a fissure; the whole surface of which is penetrated by a moss to the depth of half an inch, and in which, by taking off thin sections, the whole plant, from its root upwards, can be distinctly examined.
As we have said that botanical characters are not suffi cient to enable us to distinguish between confervx and fibres of chlorite, we must point out the chemical tests which Dr. Itlacculloch employed for this purpose. By distilling the chalcedony together with nitre, carbonic acid is produced if there is carbon present, and thus the presence of a plant may be ascertained ; while those con taining chlorite do not give out any. It is only necessary, in making these trials, ifthe specimens have gone through the lapidary's wheel, to boil them in caustic alkali for the purpose of removing the oil, which penetrates all these stones, and which would, of course, lead to erroneous con clusions.
Of Fossil Trees and it We purpose here to omit those fossil substances of this character, which are so recent in origin as to belong more properly to the division of peat. It will, therefore, in clude those only that are penetrated, or petrified by means of siltea or lime, or that are pyritized, or, lastly, that are bituminized; the latter forming that class of substances sometimes called brown coal, wood coal, Scc. but, more properly, lignite.
The existence of fossil trees was known at very remote periods, even to the ancients, as they are described by Theophrastus, Strabo, Eratosthenes, Pausanias, and Pliny. In more modern times, but before these studies had made much progress, they were known to Agricola, who de scribt s various instances of their occurrence in different parts of Germany. Some of these, as he relates, had crevices in them containing pyrites ; others, found near Cracow, were cut into whetstones, and sent as presents to Ferdinand, king of Bohemia, by the barons who held lands under him. But he, like Kircher, imagined that these were sometimes real trees petrified ; and, at others, lusi natures. Other German authors have related the same facts, but they appear often to have confounded ordinary submerged woods and forests belonging to peat, with mere ancient and more properly fossil specimens. In some of these reports we trace now and then a good deal of that marvellous which so often pretended to have dis covered in fossils, imitations of man himself, or of some parts of his anatomy, brains, hearts, legs, and arms. Thus, it is related that, in Misnia, a beech tree was found 180 ells deep in a solid rock, with all its leaves and branches entire ; a story which is paralleled by another of a ship with all its rigging, which was found petrified at a similar depth in Switzerland.