The fragments or trunks of trees that are found in the sandstones, some of which have even occurred in an erect position with portions of their roots and branches, seem to be purely terrestrial, although it is not very easy to under stand how they were submerged, and thus entangled. They seem to appertain specifically to the coal strata, nor indeed do they add much to the mystery in which these de posits are already involved. To these must be added the remains of trees, which, in many places, as near Madras and in Antigua, are found filled with chalcedony, or con verted into it ; often also accompanying remains of marine origin, similarly imbedded in alluvial soils, or lying loose on the surface.
The last subject in this division relates to the tender vegetables occasionally found entangled in chalcedony, and producing specimens commonly known by the name of moss agates.
It is sufficient here to have mentioned them in this gene ral enumeration, as we shall have occasion to describe them more particularly in treating of the vegetable remains hereafter.
Of the Modes in which Organic Remains are found The various kinds of organic fossils, of which this sketch has thus been given, are found disposed in various ways, and are also modified in different ways, as to their mineral nature. Some, indeed, are found in a state which may be considered natural; or one in which their organic compo sition has been little more changed than it would have been by the state of death ; while others are more or less converted into the nature of certain mineral bodies. A brief account of these conditions will be useful, before we proceed to consider the bodies themselves, according to their several relations as parts of the living creation.
In loose soils, or in sand and gravel, shells are often found in a dry or fragile state, appearing almost as if they had been calcined. In these the animal matter is partially or entirely dissipated in some cases ; while in others it not only remains, but even the very colours of the interior surface are preserved. In the same way, the teeth and bones of animals are often found more or less perfect, or with the animal matter preserved in different degrees.
In some rocks also, the absolute matter of the shell or other substance is sometimes found imbedded, and in such a manner as to form an integrant portion of the rock itself ; while in these cases also similar variations may be observ ed in the nature of the fossil, or in its integrity of compo sition as well as of form. In some instances even the most delicate parts are preserved in a perfect manner ; while in others again, the shells show marks of injury, being com pressed, or broken, or otherwise changed from their natu ral appearance. In a similar way, some appear to have
lost every thing but the earths which belonged to them; while in other cases even the ligament of the hinge has been preserved. In the noted limestone of Carinthia, the preservation of the internal pearly coat of the shell is so perfect, that it possesses as much brilliancy of colour as it could possibly have had in its natural state. In the same manner, fishes sometimes preserve not only their bones but the softer animal matter, as happens in Italy ; but there is no instance of which we are aware, where any large quad ruped has preserved any thing but the bones, except in the rare case of having been inclosed in ice. As to vegetables, they are sometimes found in a sufficiently natural state ; but this happens chiefly in the case of the coal strata and the lignites : the moss agates form so rare an instance as scarcely to deserve mention.
Although any substance of this nature cannot in strict ness be considered an organic fossil, unless the very mat ter of the original vegetable or animal is present, there can be no impropriety in ranking under the same head what are called casts and impressions. Though these have been distinguished by some writers, they are virtu ally the same. In the one case, if it be a shell, the mark of its concavity or convexity is left on the stone that includes it ; and if complete, such an impression would be the ex terior mould of the whole. In the case of leaves, or thin vegetable substances, there can be no other impression than this. But in shells, the form may be preserved in stony matter deposited within as well as without ; and the shell itself thus becomes the mould, while the stony copy is the cast. The matter of the original body in these cases has sometimes entirely vanished without our being able to discover how. In others it has entered into an union or combination with the earths of the rock, and this is most frequently the case in the calcareous ones. The animal matter, in the case of shells, has thus sometimes entirely disappeared ; while in other instances it is diffused through the including rock, so as to produce, in the calcareous strata, bituminous limestones. In the vegetable remains of a hard nature, as in the case of the scaly stems found in the carboniferous sandstones, the whole substance of the vegetable has thus sometimes disappeared, its form alone remaining in the shape of a cast, or an impression,or both. In the case of leaves the same occurs, and the bitumen or carbon is diffused through the shale or limestone, forming also bituminous limestones in the one instance, and in the other bituminous shales.