It has been said by Cuvier, that the fossil bones of these fissures are never found in caverns, any mere than those which belong to the alluvial strata. It is doubtful if the state of our knowledge will allow us to pronounce thus generally on this subject, nor is it easy to see what con clusion is to be drawn from this generalization, supposing it to be correct. That it is not very correct is probable; as it appears that the fossil bones of the hyxna and rhino cerous have both been found in caverns as well as in al luvia. Whatever may be the truth or value of these re marks, they seem rather to relate to the habits of animals than to any geological questions. To a British reader, the most interesting examples of animal remains found in caverns, are those which have lately occurred at Plymouth and in Yorkshire, which we formerly noticed. Whatever difficulties there may be in explaining these facts, they seem rather to be dependent, as we formerly remarked, on the habits of the living animals, and on other accessary circumstances unconnected with geological revolutions; and similar, probably, to the far more noted case of Bay reuth.
We have already had occasion to mention the animals that have been found in these places. But the general questions which these cases involve, are chiefly interest ing as they relate to the history of the animal tribes that seem to have inhabited many parts of the earth, before revolutions among themselves, or in the climate, or in their food, or, lastly, before the progress and power of man changed the face of creation. They are interesting, as they imply a great lapse of time between the period of their original possession of the earth and of their final ex tinction. But as there is nothing more of fact to offer, and little of what is reasonable to conjecture, on these subjects, and as they arc not questions that possess an in terest strictly geologicalitcannot be necessary to dwell any longer on them.
On the Intermixture of Marine and Terrestrial Remains.
The mixtures of marine and of terrestrial animals, whe ther terrene or aquatic, which are found in some places, offers another of those geological questions connected with organic fossils, that requires to be examined in this place; though much of that which appertains to it has inevitably already come under review in the examination of other sub jects in this department. It has already been fully ex plained how that may happen in the case of the alluvial de posits, and how fallacies may arise in some of these in stances; so that this part of the subject requires no fur ther examination. There remain, however, two other
'cases; namely, that of the intermixture of these bodies in the same stratum, where they are apparently of the same common period, and that where marine and terrestrial strata alternate, a general account of which will here be re quired.
When the fossil bodies contained in the same stratum are Shelly only, and though they should even contain fishes and the remains of amphibious animals, we must remem ber that the first point to be proved in this case is, whether some of these do truly belong to fresh water and others to salt. The genus of a shell is not a test of either, as the same genera are found both in fresh and salt water. Neither can any reliance be placed upon delicacy of texture, often resorted to as a means of distinction ; since some marine shells are very tender, and there are fresh water shells, such, for example, as the Mya pictorum, which are exceedingly strong. Indeed, taking the magni tude and the texture together, there is scarcely any shell more fragile than that marine one the Argonauta argo. In the ease of the more ancient fossils, it is evident that the grounds of judgment are even more feeble than in the In these eases, where the species are actually known, the distinction may be proved ; it may possibly be so, even in the genera. Yet we shall presently show that the same species in some eases inhabit fresh and salt water indifferently, that even some fishes do the same; and possibly that many more, if not all, may be induced to do so.
Respecting the amphibia, as many species of turtle are truly marine, more ancient animals of this division, and of other genera, may have been the same ; so that such am phibious beings have lived between the land and the waters of the sea, not between that and fresh-water lakes or rivers. In the Palaos islands, indeed, formerly alluded to, there is a crocodile or cayman that does leave the land to fish in the sea only, and which is therefore a marine amphibious ani mal, like the sea-turtle; so that we have no cause for sur prise because the remains of crocodiles are found in the lias.
N evertheless, the intermixture is, in some cases, un questionable ; since, even in this stratum, the predomi nant fossils of which are of marine origin, fragments of terrestrial plants are found. It is not difficult to explain these cases by means of the ordinary transportation which deposited the materials of the rock ; as the substances in question are such as might have been so transported and buried.