But not to dwell on this more than is necessary, the general fact, even taking the whole world together, is in disputable. Thus it may fairly be inferred, that while the siliceous and argillaceous strata have been formed from the ruins of more ancient rocks, a large part, at least, of the calcareous ones, is the produce of animals. Thus also it must appear, that from the operations of animals, the quantity of calcareous matter that is deposited in the form of stone or mud is always increasing ; and that, as the se condary series far exceeds the primary in this respect, so a third series, should one hereafter arise from the depths of the sea, will exceed the last in the proportion of its calca reous strata. It will combine the ruins of the last lime stones with the spoils of the present animals ; animals, of which the generations are also probably enlarging and extending in every age, in a ratio proportioned to the in crease of these calcareous deposits which they affect and favour. Those whose expanse of imagination induces them to extend the prophetic eye of philosophy to worlds yet unborn, may also thus anticipate a constant and steady approach to that universal state of fertility which is the pride as it is the character of calcareous soils.
If we now turn our eyes backwards to the primary lime stones for another purpose, we shall find in their dispro portion to the other rocks a confirmation of this opinion respecting the important agency of living animals in the production of calcareous strata. It has always been said by geologists that DO animal remains existed among the primary calcareous strata; and hence, as well as for other reasons, the transition class was invented. But even if we admit these, together with the primary, into one class, the animal remains in them will be found to bear a dispropor tion to the rocks in which they are imbedded, not dissimi lar to that which the limestones themselves do to the sili ceous and argillaceous strata. This is just what we might expect from the comparative rarity of these animals in the ancient ocean.
It has even been supposed by sonic geologists that all the calcareous strata, of whatever age, were the exclusive produce of animals. As a possibility, that opinion is coun tenanced by the phenomena of the coral islands, which we shall presently describe ; though the accessory causes aris ing from the decomposition of previous limestones, must unquestionably be admitted, as far as relates to the secon dary strata. But the mere existence of primary limestones thus operating, by their destruction, to assist in producing new ones, is no proof in itself that even these are absolutely original and independent of animal sources. Such rocks are in themselves rare, and they are so situated among other rocks that have undergone great changes of various kinds, that the organic remains, once imbedded in them, 'may have been obliterated. That something of this na
ture does actually happen, even in the secondary, is cer tain, as Dr. Macculloch has proved in the island of Sky, where a series of secondary calcareous strata, containing shells, is at different points converted into a crystalline unstratified limestone, in which all these remains are ob literated, and where the very gradation of the process can also be traced, as it may equally be in the Isle of Man. The causes which have been assigned for these changes have been pretty clearly demonstrated by him ; but we shall not pursue this part of the subject, as it would lead us too far from the present object.
In concluding these remarks on the power with which shell-fish have operated, and are now probably operating, in the production of calcareous strata, we must add, that we do not mean to assert that all limestone, or all calca reous earth, is the produce of animal action. On the contrary, we are quite sure that it is an ingredient in the composition of the Earth, in circumstances where this cannot be admitted. It is moreover probable, or rather certain, that, so far from producing it, they merely dispose of it by secretion, having first acquired it in some way yet unknown to us, from the element in which they live, so that, however much they may have fabricated limestones, they have not generated them, but merely acted as chemi cal and mechanical agents in this most important and in teresting work.
We must now return to the corals, because, in the ope rations of these animals, we find that which we cannot in those of the shell-fish. In the latter, we can only infer from observation and analogy, that the immense masses of our present calcareous strata have thus been produced. We transfer from the bottom of the sea, those operations which we knew to be daily going on ; and, reasoning from them, recur to a time when our limestones were in the same act of being formed, and were preparing for future dry land ; land to be laid dry by its own elevation, or by the receding of the water, as geologists shall hereafter agree or prove. But there is a perfect and complete chasm be tween the two, at least in the case of marine strata. In the terrestrial or fresh-water ones it is otherwise, as we can follow the marly deposit of alake, till it rises to the level of the water, and, gradually excluding it, prepares the dry land ; an operation, of which every country, and our own mountainous region, as distinctly as any, allbrds the daily proofs, in the marl deposits covered with soil and peat, that are found throughout the Highlands of Scotland.