It only remains to inquire if organic remains have been found beneath such rocks, that we may try to ascertain to what relative period of the earth the existence of animated beings can be traced. It seems to us that geologists in general, proceeding on some preconceived opinions res pecting this subject, have been less industrious in their en deavours to elicit the truth on this point than they might have been ; and for this reason, chiefly, it seems that scarcely any information can be procured on this head. Such as it is, if not positive, it is at least worthy of consi deration, and we must therefore state such facts as can at all be brought to bear on this obscure and neglected subject.
A particular kind of coal, either anthracite, or a sub stance approaching to it, has been found in the very situ ations in question ; and although we have no right to conclude absolutely that it has originated in vegetable sub stances; that circumstance is rendered highly probable by the origin of the analogous substances in the secondary strata, while it is also not a little confirn.ed by the manner in which this primary coal is distributed in these remote rocks.
In the next place, as we can only here proceed on faint probabilities, it has been remarked by Dr. Macculloch that, in the hornblende schist of Glen Tilt, which alter nates with gneiss, primary limestone, and quartz rock, there are found imbedded calcareous portions or fragments not exceeding an inch in dimensions, uniformly thin, and concave. In any situation but this there would be no hesitation in considering these as fragments of shells, and perhaps we refuse to admit them to be such in this case rather from our prejudices than from any good reason. If any other examples of this nature should be discovered, they will possibly serve to prove what it may be thought they scarcely now render even probable. The last in stance of this nature was already noticed, and it depends on the same observer; who, in his remarks on it, fully aware how unlikely it was to be true, and how cautious we ought to be in receiving it, assures us that he spared no pains to annul his own conclusions, by a careful and critical examination of the rocks, and their relative posi tions. The quartz rock in question, already mentioned, is not only interstratified with gneiss, according to the in clination of the strata, but lies under an enormous series of consecutive strata of that and other substances.
It is not to be expected, on the most favourable view, that organic substances, animals at least, should be found in quartz rock, because even in the secondary strata they do not often occur in the rocks of this nature; yet the last mentioned instance is an exception. But Brongniart in forms us, that, in many parts of the Appennines, shells are found beneath serpentine and diallage rock ; a case of some difficulty, it must be admited, although, as the lime stone in question which contains these remains is consi dered a secondary stratum, it must be supposed, however at variance with the received opinions, that both these rocks also belong to the secondary class.
If it be true, as most geologists think, that the greater part of the secondary limestones have been formed by ani mals, it is probable that these also have had a share in the formation of the primary, or the ancient calcareous strata. Hence we might expect, that fossil remains should be found imbedded in these ancient limestones. We noticed this circumstance before ; and need only add, that if the cal careous rocks of Plymouth and its neighbourhood are pri mary, as we believe them to be, the fact is ascertained. As far as the present question is concerned, it will make no difference whether these are called primary or transi tion rocks : a name will not alter the fact, and the only point here at issue is their antiquity, as compAred to the superior and decidedly secondary strata. If fossil remains are not oftener found in these limestones, or if, where they do exist, their forms are mutilated or imperfect, that cir cumstance is explained by the fact already mentioned, viz. that even in the secondary calcareous strata which contain such remains in abundance as part of their constitution, there occur portions from which the organic bodies have disappeared, in consequence of the proximity of trap rocks, and their having undergone the process of fusion. Now this is exactly what might be expected to have happened in the older primary limestones that alternate with gneiss and other rocks that appear to have been exposed to the action of heat, and which accordingly do not contain or ganic remains; while where the same calcareous rocks occur higher up in the ancient series, where that has not been exposed to heat, such organic remains are found.