If we compare the inorganic rocks of the primary and secondary classes together, we shall find that there is a correspondence in their general proportions, as far, at least, as their chemical nature is concerned. That is to say, if there is abundance of granite and gneiss among the pri mary, and none among the secondary, there is a perfect chemical compensation in the sandstones and shales of the latter. The quartz of the granite and gneiss corresponds to that of the sandstone, and the clay of the slates to the felspar and mica of these two abundant rocks. We might, if it was necessary, or competent with our limits, to be here minute, draw this conclusion much more widely, by enumerating all the rocks of the primary series, such as clay slate, mica slate, and quartz rock, and comparing them with all the siliceous and argillaceous strata. That the one set of these has been produced from the destruction of the other, is supported by facts so universal and incontro vertible, that no rational geologist entertains a doubt on the subject. We cannot, however, enter into the proofs, as these would lead us far beyond the prescribed bounds.
With regard, now, to the possibility of the secondary limestone having been, in a great measure at least, the produce of shell-fish, we shall hereafter see, in the descrip tion of the coral formations, that there is nothing in their magnitude, at least, any more indeed than in the nature of their composition, to prevent us from believing it possible that they might have been entirely formed by these ani mals. Were we even to suppose, that every particle of the largest bed of limestone that is known was originally part of the body of a shell, we should, as far as the bulk of the mass is concerned, assume nothing that would not be countenanced by the magnitude of the great coral reef of New Holland, which will be described hereafter. If the most minute animals of the creation can thus, by their numbers alone, execute, unassisted, works of such enor mous magnitude, and, as navigators think, within spaces of time comparatively limited, it is far from unreasonable to imagine, that the succession, through unnumbered ages, of animals so far exceeding them in bulk, and in the rela tive quantity of their calcareous produce, should have generated all the calcareous strata in the secondary series.
It is not necessary here to ask, whence the calcareous matter has been derived, or whether it is an animal pro duct. The difficulty as to its production is perhaps at pre sent insurmountable, but, in this case, it is of no moment. It can form no objection to the power of oysters or pec tines, in producing, by their own energies, a bed of lime stone ; because the fact, however inexplicable, is rendered unquestionable by the generation of coral from sea-water. That very extensive beds of calcareous matter may be pro duced by animals and from their remains, is also incontes tibly proved by those deposits of shell marl, so frequent in fresh-water lakes, as well as by the recent oolites of the West Indies, and probably by our own ancient ones ; as these seem all to have been equally formed out of commi nuted shells alone, As to fresh water marl beds, in many cases, in the Highlands of Scotland, we can decidedly prove that this is their sole origin ; because we can trace the streams by which the lakes have been fed, along their whole courses, and thus ascertain that they could not have carried down calcareous matter ; their origin and progress being among siliceous strata.
We do not, however, mean to deny, that if calcareous beds are now forming at the bottom of the sea, they must be formed, in some measure, as the shales and sandstones have been, from the waste of the present calcareous strata there deposited ; and thus the operations of shell-fish will only form a part of the causes of their production. Nor need it be denied that such has been the case, to a certain degree, in former times ; but that the assistance afforded by the ruins of primary calcareous rocks has been very trifling, will appear evident from a simple arithmetical computation that cannot deceive us.
Every thing, as we just hinted, proves that the present secondary strata are the produce of more ancient rocks ; and these must have been a part of what are at this present moment our primary, as we have no reason to imagine that there has been a distinct series which has entirely vanish ed. The relative proportions of the different materials in the produce, ought therefore to bear certain relations to those in the original repositories ; as, if there was a diffe rence, it would be expected to be in favour of the most yielding materials, as in the schists and limestones. But if we examine the quantity of limestone in the primary strata, it will be found very small. What the exact pro portion of this to the other rocks throughout the world may be, is not known ; but in Scotland and England it cer tainly does not amount to a thousandth part of the whole. But among the secondary strata of England, the limestones bear an infinitely larger proportion to the siliceous and argillaceous rocks. If we were to assume only the ratio of one hundredth, it would answer the purpose of the pre sent argument, and there is nothing unreasonable in refer ring the origin of the British secondary strata to the Bri tish primary rocks. We might easily extend this argu ment from the comparison of localities and proportions in the primary and secondary limestones, to all parts of the world, but shall here content ourselves with pointing out the enormous secondary calcareous ridges of the Ap penine and the Jura, and by desiring our readers to com pare their extent with the very limited quantity of primary limestone that is found in the Alps.