When we treat hereafter of the vegetable organic re mains, we shall, under the head of Lignite, examine par ticularly this matter, as far as it relates to the vegetable re mains supposed to he included in trap. But we must here notice the cause of the error, as it relates to shells. In particular situations, where masses of trap cover beds of shale, or penetrate them in the form of veins, such shales are observed to be so indurated as even to strike fire ; be coming what is called siliceous schist, and, if very black, glossy, and compact, Lydian stone. This appearance is common in Sky, where it is fully described in Dr. Maccul loch's work on the Western Islands, and it is also known to occur in the north of Ireland, and in many other places. Now shells thus indurated have frequently the aspect of basalt so strongly, as to be easily confounded with it by a careless observer : and this error is the more easily made on account of the contact and intermixture of the differ ent substances. Now it is in these indurated shales or siliceous schists, that the shells are found, and not in the superincumbent basalt ; a circumstance perfectly intelli gible, because the common soft shales with which these are connected, and of which they are modifications, are among the ordinary repositories of these shells.
The reason why the trap rocks ought not to contain or ganic remains, is found in their igneous nature ; since those, at least, which have been enumerated, are the produce of fusion. But for this reason also, we can understand why such bodies may exist in the tufas or conglomerates. Either these have been alluvial deposits of gravel, over which the fused rocks have been spread, as lavas are over over ordinary alluvia from existing volcanoes, or else they are the produce of erupted fragments of stones and dust, or what are commonly called ashes, in the case of volca noes. Now, in either of these two cases, the trap tufas might, without difficulty, include organized substances. An instance precisely analagous is pointed out by Faujas de St. Fond, where trees were found covered by solid vol canic rocks of lava ; and the presence of the remains, in the one case as in the other, is no more surprising than their existence in ordinary alluvial soils. The other case of erupted tufas may be understood from the condition of many parts of Italy. If the very works of art of Hercu laneum and Pompeii cotad thus be buried under erupted matter of this nature, the same might as well happen in the case of the tufas of trap ; as we are by no means in clined to admit that all the trap rocks are, as Hutton's the ory supposes, of submarine origin. And such cases as this may as readily be conceived as those which are so com mon throughout 1 taly, where terrestrial remains, aquatic or not, are frequently covered with various kinds of tuface ous rocks.
Having cleared up this point, we may therefore return to the stratified rocks, and inquire among what species of these, organic remains are found imbedded. Wherever they do exist, they are sufficient to prove that the rocks have been deposited from water, although their absence from any particular rock does not prove the reverse. It is quite possible that many aqueous strata have been formed before the creation of such beings. It is not only possi ble, but probable, that many strata have undergone such changes as must have been sufficient to destroy such sub stances had they oven been originally present. There are others, besides, which do not appear to have offered, when in their unconsolidated state, habitations fitted for creatures provided with instincts to choose convenient seats for their propagation : and lastly, these bodies might not have been present in the living state when the strata were originally deposited. We have no reason, indeed, to suppose that the whole bottom of the sea was, at all times and places, covered by colonies of shell-fish ; since, even at present, that is not the case. For these reasons, therefore, there is no cause of surprise, if, as is before observed, they are not found in gneiss and the older sehists ; there is none, even, that they are not found in all the secondary strata. It is easy to conceive how, from one or other of these causes, they might be present in a given bed in one place and not in another ; how they might be found in one bed and not in the succeeding one ; how, among alternating beds of dif ferent rock, they might be attached to the argillaceous or calcareous, and not to the siliceous strata ; and how, lastly, they might exist in one calcareous stratum and not in another, even although in the same neighbourhood.
It is very easy to account for the presence of organic remains in those strata in which they are found, when we recollect the origin of these, and when we examine mo dern oyster banks, the marl deposits of lakes, the remains so often found in gravel and in peat, and the vegetables which are intermixed with alluvial clay, or with the mud of lakes and of sea-shores. During the consolidation of these substances, the shells, heaped on each other in the order in which they lived and died, have become portions of the stone ; while, where they are found broken and in termixed with arenaceous rocks, they remind us of the manner in which they are to be seen in our own sea shores in beds of loose sand. Our English Purbeck stone fur nishes an appropriate instance of this modification.