Now, although in all such examples the fossil itself cannot be said to be p esent, the same purpose nearly is answered by the casts and impressions, and, indeed, were we to exclude these, we should have to resign a great many species, particularly in the vegetable kingdom. In the cast of leaves, in particular, we have often nothing but the impression, as, indeed, is equally true of the thicker stems ; while there is still a better reason for making no distinction, or rather exclusion, in the fact that the grada tion between a mere cast and one containing some por tions of the original organized matter is such, that there is no point at which we could stop were we inclined.
The next state in which organized bodies are found, is that which is commonly called petrifaction. This presents a great variety of appearances, both as to the substance, or petrifying matter, and as to the degree in which it has taken place. In those cases where the process is perfect, the original matter of the body in question has entirely disappeared, but the organic structure is more or less per fectly preserved in the substitute. This is often the case, for example, in silicified wood, as it is also in some corals petrified by the same material ; the delicate organization of both appearing to have been entirely converted into flint or chalcedony. In other instances, however, the process is incomplete; and while some parts of such a body may have entirely disappeared and have been replaced by the petrifying substance, in other portions of it parts of the original matter still remains. Thus in silicified wood such parts are found to blacken sulphuric acid ; the woody mat ter being still sufficient to decompose, and to be decom posed by, that substance at the same time, while they also yield carbonic acid by a proper mode of distillation. Thus also in silicified shells, it is sometimes observed that not only the calcareous earth of the shell is combined with the flint, so as to convert it into a chert, but that even these, by acting in a similar manner on that acid, prove that they still retain a portion of the animal matter of the ori ginal body.
These petrified bodies are either of a vegetable or ani mal nature, as already remarked ; and they are found loose in alluvial soils, as happens frequently in the case of sili cified wood, as well as in shells and corals. In other cases they are included in solid rocks, either of the same nature as the petrifying substance, or of a different one. Thus calcareous petrifactions and siliceous ones both may be enveloped in chalk ; and thus bituminizcd vegetables may be inclosed in coal or in shale.
The petrifying matters are various, and among them we may include what are called metallizations. In the case of shells, carbonate of lime is one of the most common ; but in these also, as well as in vegetables, the petrifying substance is sometimes indurated clay, viz. shale or schist.
Silica, under various modifications, is also common. Thus echini and other marine bodies are preserved in flint. Chalcedony, another modification of that earth, is also found to have penetrated wood, shells, and corals ; an oc currence common in Antigua; and besides these there oc cur those varieties of chalcedony, if they may so be called, which have been named opal, semiopal, cacholong, jasper, and agate, all of which are occasionally found to be the petrifying substance of wood and other vegetable matters. If quartz is more rare, it still occurs in certain cases. The metallizations are nearly limited to pyrites, which is by no means uncommon in the case of shells, but is very rare in vegetable petrifactions. Something of the nature of bog iron ore, or the hydrated carbonate of iron, has also been found performing the function of a petrifying substance; and though it has been said that sulphuret of mercury has also been observed in some fossils, it is far too rare a case to require particular notice. To all these we must, how ever, add bitumen; as, although it is not a petrifying mat ter strictly speaking, it performs the same office in the case of many vegetable remains. The conversion of vege table matter into charcoal, or into that compound of car bon and hydrogen, which produces those beautiful brown pigments called Cologne earth, can scarcely rank among any of these varieties. Both these subjects, however, will be more particularly examined hereafter in treating of the lignites.
The last thing in this subject to he mentioned, is tlit is popularly called petrifaction, but which, strictly speaking, is incrustation. These are generally of recent origin, and very often scarcely worthy of being ranked among fossil remains. In this operation the organic sub stance is incrusted with the stony matter, with little change of its own character, and without much penetration of the earth into its texture. Of the few instances which are more particularly deserving of attention, on account of their geological nature and connections, are the fragments of vegetables and fresh-water shells entangled in the tra vertino of Italy; and in that country, also, such substances, incrusted with calcareous matter, are sometimes met with in the volcanic tufas. It is necessary here, however, to give Signior Brocchi's explanation of what might else ap pear somewhat difficult : and he has shown, that all such tufaceous deposits, however volcanic in their materials, are alluvial rocks, having been moved from their original places by water, and consolidated from the cementing property of the puzzuolano, or volcanic dust which they contain.