Of the Geological relations of the rocks that either contain organic remains or may include these beneath them.
As a general, if not an absolute order of succession has been observed to exist among stratified rocks of different kinds, and as, in some districts, that order has been ob served, under certain modifications, to be regular, it has been supposed that some similar or analogous order exist ed among the places, numbers, and tribes, of the fossil bo dies which they contain. Two points of some consequence arc connected with this opinion ; the one relating to an absolute order of succession, or, as is thought. of creation, or existence of the different races of organized beings, and the other to the constant or necessary connection of par ticular species or genera with certain corresponding strata. The latter question must be reserved for a full examination at a future period, and the other will also be examined in its most appropriate place. But as the na ture and succession of the different kinds of strata them selves, and the presence of any organized bodies in them, are more properly connected with the subject examined in the last division, it is preferable to consider that part of the question here.
To examine, now, in the most general manner, the suc cessions of organized bodies merely as they exist in the strata, it must first be remarked, that wherever we may choose to commence among the upper ones, there is not an uninterrupted succession of these downwards to the point at which they cease to be found. This might indeed have been deduced from the remarks already made on the na tures of the strata in which they do, and on that of those in which they do not exist ; but it still requires to be stated in greater detail. Many strata abound in them when they arc wanting in those immediately above or below ; although it is a general rule that they are most numerous and vari ous in the upper beds, and that, at a certain point in the order downwards, they either materially diminish or en tirely disappear. But as it was already shown that these bodies were chiefly prevalent in the secondary strata, and among these, principally in the shales and limestones, while they never occurred in the unstratified rocks, the present subject is, on certain points, anticipated. It re mains, however, to describe more particularly those rocks, under which, as well as in which, they have been found, and those whence, in the same manner, they are absent.
No instance, as we already remarked, has yet been pro duced, of an organic substance in gneiss or micaceous schist, as well as in some other rocks that need not again be enumerated. Neither is this possible, at least in the greater number of these cases, if, as many geologists sup pose, these strata have undergone the action of a high heat. But as there are instances of micaceous schist, con taining mechanically rounded nodules or fragments of other previous rocks. and being thus conglomerates, it is within the limits of possibility that they might also con tain shells ; supposing, what is a necessary condition in this case, that such animals were really in existence before they were deposited and consolidated. If, as many per sons think, micaceous schist is purely of an aqueous ori gin, there would be no difficulty whatever in this, more than in the case of micaceous secondary sandstones, to which these rocks are so analogous in their composition ; it would not be impossible, though it should have under gone some change of texture from the action of heat ; be cause we know from experiment, that shells can be expos ed to a high degree of temperature under pressure, with out being necessarily destroyed. Yet, under such a view of the possible or probable nature of these rocks, such an occurrence, though it might exist, could not be frequent.
If, therefore, organic remains are not found imbedded in these rocks, and even if that should never happen, it does not prove that they were not in existence before these were produced. It is easy in explain their absence, from other causes, and principally, from that just alluded to, namely, the high temperature to which they may have been exposed. In addition to this, it must be observed, that even in the secondary strata, where such fossil re mains abound, they are generally absent from rocks of analogous character, that is, from the sandstones : evi dently because the earths or materials of which these are composed, did not furnish them with commodious or fa vourite habitations when living. It is also not improbable, that in the earlier states of the globe, in which these low est rocks were produced, these animals were far less abun dant, although not absent; a fact which is perhaps evinced by the scarcity of primary limestone compared with se condary; since it seems clear that a very large portion, at least of the latter, is the produce of animals.