The stones of this imagined nature, commonly called Mochas, but properly moths, (in German) or moss-stones, are very well known. In these there are black arboriza tions, which the most superficial examination will show to be utterly void of the vegetable structure, and which arc exactly like those so often found in the fissures or sub stance of rocks, as in some of the schistose marls, and in cherts.
But the most common cause of deception, in these cases, is a peculiar fibrous and ramified, or contorted dis position, often assumed by chlorite. On a careful exami nation of this substance, as it is sometimes found in quartz crystal as well as in chalcedony, it may be observed that the successive hexagonal scales of the mineral are repeat ed in such a manner as to form a fibre; which may vary in size, as well as in length and in disposition. Thus it may be simply contorted and entangled, or even ramified ; and in all these cases, particularly in the latter, it is easily mis taken, as it has often been, for a conferva. Where the mineral is of its natural green colour, as is very common, the deception is very much increased ; and is indeed often such, that it is no wonder that agates of this aspect have been supposed to include real confervas. In other cases, however, instead of retaining its green hue, the chlorite is sometimes decomposed or rusted ; and in some in stances, again, it appears invested with a coat of iron, in some state of ochre, which is sometimes yellow, and at others red. It is by these that all the red and yellow fibrous moss agates are produced ; and it is from know ing those only, that so many mineralogists have denied the existence of real plants in chalcedony. At the same time they are infinitely the most numerous, being, in fact, extremely common, while the true specimens are as rare. In England and Scotland they are abundant on the shores of Scarborough and about Dunbar ; and it will be found, that it is by these very specimens that the doubts just mentioned have been excited. As owing to the darkness of many of these stones, they will not transmit light enough, even when cut to an extreme thinness, to per mit the anatomy or structure of the fibres to be examined, we must then have recourse to the chemical methods which will presently be pointed out.
Where real confervx have been thus found, it is re markable, not only that the vegetable forms are perfectly preserved, but that the plant, however light and yielding its texture, is disposed in as free a manner as if still living and floating in its native element. The colour is also pre served in most cases, so that the greens arc as bright as in the living vegetable. But it is still more interesting to observe, that these colours vary from the most brilliant green to the darkest sap or lightest yellow green, which at present characterize the different living specimens with which we are acquainted. Still more is it worthy of re mark, that certain parts of the fibres sometimes present the yellowish and reddish, and even the dirty white hue which the plants themselves so often display after death : these appearances being very characteristic of their vege table nature, as they could not be imitated either by chlo rite or any of the metallic oxides.
It is scarcely necessary to say that, although the general habits or characters of these plants can thus be ascertained, it is impossible to make out species among them. In the living plants, that is difficult enough, as botanists well know, on account of the delicate circumstances by which the different conferve are distinguished. It is impossible here ; because not only in cutting the stone thin enough to allow light to pass through it for this examination, the form of the plant is destroyed, but because the natural opacity of chalcedony, with all the contrivances which we can employ, is such as to impede the passage of sufficient light.
In some of the larger plants this can, however, be done. Daubenton professes no doubt whatever respecting some of the species which he has examined ; and as he describes particularly the Lichen •angiferinus and digitatus, it is impossible that he could have committed any error; since no mineral could imitate plants of such remarkable forms. Dr. Macculloch has also described and figured an imbri cated lichen, akin to the centrifugus and saxatilis in its general habits, but still more strongly resembling the Par melia Borreri.