There are many varieties of silicified wood. Some times it appears to have been in a rotten state before it was penetrated by the stony matter, so that it is tender as well as hard and brittle. In other cases it appears to have been first bituminized, so that it is both bituminous and siliceous together, in which instances it is frequently of a dark or black colour. Lastly, it appears to have under gone no previous change ; and in some of these cases the wood, or at least part of the ligneous matter, remains in the petrifaction, while in others the whole appears to have vanished, leaving nothing but its form behind.
The siliceous matter is found in the different states of chalcedony, agate, jasper, and opal, as well as quartz ; but between the three first the differences are rather no minal than real. Colour is sometimes here made a ground of distinction; at others these are derived from the more or less perfect state of silicification, and thus they are compared to one or other of these minerals when in their simple state. The opaline woods are among the most remarkable, as well as the most beautiful : but our limits will not allow us to enter into more minute details on this part of the subject.
Wood is frequently found petrified by carbonat of lime, and the quarries of Portland have produced large trunks of this nature. It has also been observed in the Mendip hills, in Oxfordshire, in Yorkshire, and in other parts of England, as well as in many different parts of the world. In these cases, however, we must distinguish between the most modern, which are rather incrustations, such as those of Matlock and other places, the next most ancient, such as those found in the Italian travertino, and those still older, which belong to the secondary strata. The first kind is extremely common, the last is comparatively rare.
The state of these is as variable as that of the silicified woods. Sometimes they seem to have been rotten before the change took place; at others they are both bituminiz ed and filled with calcareous matter at the same time ; and in other cases, again, they are perfect stone, with more or less of the original woody structure remaining. These various circumstances are easily ascertained by inspection, and by chemical trials.
By far the most remarkable instance of calcareous wood that has been produced, is that discovered by Collins in New South Wales. On a sandy spot, a hundred feet above the level of the sea, he found a quantity of broken branches of dead trees of two or three inches in diameter, intermixed with stumps of similar size, standing in an erect position, and shooting their roots into the soil. They were extremely brittle : and, on being broken, the various structures of the wood were quite discernible. The ex
terior part resembled a calcareous sandstone, and in some the interior was like chalk. The wood was partially pre served in some of the specimens ; but in others it had en tirely vanished. This appearance is very inexplicable ; but a fact somewhat analogous was observed near Far ringdon, in Berkshire, where part of the root of a living elm was thus found partially covered, and penetrated with a calcareous incrustation.
It has been said that wood has been found penetrated by aluminous clay or schist, and thus capable of forming effiorescences of alum. It is of a brown colour, but not heavy ; it contains pyrites, of course, and thence it some times takes fire spontaneously. This species is found in some parts of England, in Bohemia, Austria, Hesse, and Saxony. Pyritous wood, with little clay, but, in these cases, generally also containing bitumen, is not uncom mon in many places. Nor is it very unusual for pyrites to be found in sornerof the silicified woods.
Other metallic impregnations of wood have been ob served, but they are still more rare. A large tree was found in Siberia, converted into iron ore, but the particu lar state of the metal is not explained. According to Mr. Tooke, the same appearance occurs in Russian Lapland, where the trunks, branches, and even the leaves of birch trees are converted into an iron ore ; which seems, by the account, to be a mixture of pyrites and bog ore. Phos phate of iron sometimes occupies the places of the roots of plants in different parts of England. In Siberia, there occurs wood penetrated with carbonate of copper, or with a mixture of that salt and carbonate of lime. The speci mens are sometimes blue, and at others green.
It has been imagined, by many observers, that they could ascertain the nature of the wood forming these pe trifactions. But this is often more than doubtful ; al though the botanical affinities have certainly been some times discovered when the species has not. Among these have been named the pitch fir, the larch, the hazel, oak, ash, alder, box, beech, sandal, laurel, willow, mulberry, vine, manchineel, tamarind, palms, and many others. It is not worth our while to attend to these statements, often the results of inattention or fancy. As little need we no tice the supposed recent petrifaction of wood shaped by art ; such as is related of Trajan's bridge across the Da nube, and of ladders and buckets found in mines ; as these seem all to have been instances of modern incrustations ; in a certain sense as artificial as those daily made at Tivoli as substitutes for plaster casts.