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as since most indisave been forward in this count who
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he had undertaken; "It is scarcely necessary to inform our geological readers of the oblivion into which that work has deservedly fallen. The ardour with which this particular branch of science has since been cultivated, had led us to hope, that the blank in this most indispensable part of its elementary knowledge would, ere this, have been supplied by some one of those who are now ardently pressing forward in this course in Britain. Nothing however has yet been done in this country, and it is chiefly with a view to excite the industry of those who may be possessed of the information required for such a work, that we are induced to notice the present compilation,
It may perhaps appear extraordinary to our readers, that while our presses have groaned under the Systems of Mineralogy which have been produced in such rapid succession for the last few years, no arrangement of Rocks has been formed, except the abortive production above mentioned. This dearth, or rather, absence of such works, is, however, not difficult of explanation. Excepting the collection of Essays which stands at the head of this article, and some others of no greater moment, which it is unnecessary to mention, no systems of this nature have been published from which our makers of books could have borrowed their materials : And these Essays are not of a nature to admit either of being reconcocted or garbled by the compilers whose motto is . nil dictum quod non dictum prius.' There are not, on this subject, the Lectures of Werner, nor the System of Haüy, into which the manufacturer of a voluminous work may dig for his materials:he must have recourse to the great mine of Nature-a mine closed to those 6 homines trium literarum,' whose talents are limited to the art of " pouring out of one phial into another, and who, when they have transposed a few specimens from the top to the bottom of a cabinet, imagine that they have made wonderful progress in science. Let us but see one tolerable arrangement of rocks, and we venture to predict, that no long time will elapse before similar works will swarm around us; from the bulk of two or three 8vo volumes, to that of the mini, kin productions of Mr Mawe.
The present work contains the Essays of Brongniart, De la Métherie, Tondi (published by Lucas), and Brochant,-names well known to our geological readers; together with an appendix on volcanic rocks, comprising the schemes of Dolomieu, Thomson, Haiiy, and Faujas de St Fond. We shall attend principally to the four first authors, who have treated that part of the subject which is the most general and important: of the latter Essays, a brief notice will suffice. Of these four,
Well known to our pocks, comprising st Fond.)
the two first have adopted an arrangement founded on Mineralogical principles, or on the mineral characters of the rocks, whether simple or compound: while the two latter have arranged the rocks which they describe, according to the order or the analogies which they hold in nature towards each other, and to the general structure of the earth; thus adopting a GeoLogical, instead of a mineralogical principle of arrangement. Each system has its advantages, and each has its inconveniences; and as we are of opinion that the whole question of present utility and future improvement hinges upon the choice which is here open to us, we shall take the liberty of examining this part of the subject in some detail.
Our readers who are conversant with the works of these several authors, will not be surprised to learn that Brongniart alone has given the reasons for preferring a mineralogical to a geological method. These are detailed at some length in his prefatory observations. De la Métherie propounds bis arrangement without defending it; and the other two, though not with equal vigour, follow, as is usual with the pupils of that school, in the infallible track which leads from Freyberg through all the obscure regions of nature. If we shall be found to coincide with them in the principle of arrangement, it is not because, like them, we have drank of the fons Caballinus;' but because we approve of the principle which the sagacity, rather than the philosophy, of Werner, has led him to adopt.
To that sagacity, to his persevering industry and accuracy in minutiæ, we are always ready to render justice; but we must be permitted to express our doubts of his capacity for generalization, or for those wide views without which no man ever emerged from the haberdashery of experiment or observation. It has been said, that " Si Dominus Deus non fecisset Papam infallibilem, Dominus Deus non fuisset discretus ;' and the same maxim appeared for some time to be adopted by the pupils of this celebrated school. But Jack and Martin have begun to cut off the epaulettes; and we trust, in no long time, to see the reformation established on the more solid basis of extended observation and cautious generalization.
• Rocks,' says Brongniart, “ may be considered under two different views ; first, according to their composition, that is, according to the nature, the quantity (or proportion), and the disposition of the substance of which they are formed ; secondly, according to their position, or to the places which they occupy in the structure of the globe, and the analogies or relations which they bear to each other. From these considerations there result two principles of classificaţion ; and we shall proceed to consider,' &c. &c. The arguments for and against the two principles of arrangement are then briefly stated: But as we do not consider that the excellent author has examined this subject with much care or affection, we shall not proceed with our extract, but rather endeavour to lay before our readers our own views of this important question.
A classification of rocks on a Mineralogical principle must unquestionably be considered, in one sense at least, as a natu. ral arrangement; associating those combinations of minerals which are actually found in nature, just as in the organized world we associate certain combinations of forms. It may also be contrived in such a manner as to distinguish the simple from the compound rocks, and thus to refer the former to their proper places in the system of mineralogy; while it is more exclusively occupied in the classification of the latter.
As, in nature, the same compounds occur in different Geological positions, it follows, that any arrangement on a Geological principle must involve repetitions, which cannot occur in one founded on mineralogical characters. It is also evident, that from the present imperfect state of the science, much must be assumed or conjectured respecting the general order of succession among rocks, and the analogies which they bear to each other; so that, to a certain degree, every geological arrangement must be hypothetical, while no hypothesis is involved in a mineralogical one.
Were a sufficiently extensive nomenclature provided and received, it is evident that a mineralogical arrangement would furnish a name for every rock; and that it would thus be enabled accurately to limit the same term to the same compound, whatever might be its position in nature. To all which advantages it may lastly be added, that the knowledge of rocks classified on this principle, demands no geological investigations ; and is open to every one who has acquired the previous knowledge of minerals in their simple state.
Now we most readily admit, that, as far as the mineral history of the species or varieties of rocks is concerned, a mineralogical classification is the best that could be adopted ; and that it would, if perfect, materially facilitate the examination or description of a cabinet of specimens. Such a classification could only, however, be considered as part of a system of mineralogy. As yet, all these systems must be considered as artificial; classing, in the manner most convenient for investigation, those substances of which the obscure and involved affinities have as yet prevented all possibility of a natural arrangement. If the simple rocks, therefore, are to be described among the simple minerals, as in the system of Brongniart, the compound, to render the system uniform, should be enumerated in the catalogue of accidents to which each mineral is liable with respect to mixture or association. If this be not adopted to its full extent, it is obvious that rocks, objects of such importance, not only in the history of the earth, but in that of the minerals themselves, become separated into two distinct works or catalogues. As it is also not uncommon for a single mass of rock to be compounded in one place and simple in another, it would be necessary to search for it in two distinct catalogues; and possibly, in the imperfect state to which all new arrangements must be subject, in the works of two different authors.
The difficulties to which we have just alluded, which have hitherto prevented us from establishing a natural, and at the same time an useful classification of minerals, interfere even in a greater degree with any attempts to form a natural and an useful arrangement of rocks on a mineralogical basis. The most important characters of these are frequently to a considerable degree independent of the minerals which enter into their composition. Not unfrequently, also, a rock will retain all its most essential qualities, although undergoing considerable mineralogical changes, by the loss of one, or the acquisition of another substance. It must also be evident, that the capricious, and almost endless modes in which the minerals that constitute rocks are intermixed, would lead to an enumeration of species that would confound the student by its excess, or, if curtailed, defeat the object of the contriver. . · It is, therefore, an important defect in such a classification, that it bears no necessary relation to some of the most interesting characters of rocks. But it is also difficult to propose any mineralogical method which is uniform and unexceptionable; even were we to overlook the advantages derived from one which should fulfil that condition to which we have just alluded. Let us suppose, for example, that the presence of some particular mineral is made the ground of association. In such a case, it is evident, that substances most widely separated, not only in their natural affinities, but even in their characters as mere specimens, or rocks, may be associated together; since the minerals which enter into their composition are very few, and are repeated under many different combinations throughout a great number of rock species. We need not quote examples to the geological reader of the confusion that would thus ensue, for example, by assuming either quartz, mica, or felspar, as the common bond of a class, or of any inferior division.
If, again, the predominance of some one mineral be mado the groundwork of a division, it is easy to see that the same rock might be separated into different classes or subdivisions of an inferior kind. This, for example, would often happen in the case of granite ; which may contain in excess any one of the different minerals that enter into its composition.
In the next place, let it be imagined that the texture of a rock is assumed as a common bond of union, and it will be equally apparent, that under the granitic, the porphyritic, or the schistose, cognate substances may be widely separated, and those which are entirely different in other more essential circumstances be associated. The same reasoning applies to any attempt to arrange rocks according to the number of the substances of which they are composed. Thus, if a binary, or a ternary proportion be made the groundwork of any association, similar effects will follow; as it is not unfrequent for the same rock to vary in the number of its ingredients. It would be abundantly easy to illustrate all these objections by a reference to well-known rockss but the enumeration is scarcely required, and would extend these remarks too far. It must, indeed, be obvious, that any mineralogical arrangement, even should it combine all these methods in the most careful manner, must in a great degree be arbitrary; and that it must hold out the shadow, rather than the substance, of a natural method.
If we even imagine such a system to be perfected, it is evident, as we have hinted above, that it would require a very numerous, as well as an appropriate set of terms; and this, as far as it has been executed by the able author who stands first in the work under review, it actually does. Let us consider, then, how this would affect geological descriptions, the principal obs ject for which a knowledge of rocks is required. There is no necessary relation between the composition of a rock and its place in the order of nature; and many varieties of composition, as we have just seen, occur in the same mass of rock, as in the familiar instances of the porphyries and gneiss. Many terms would, therefore, be required in such cases, to describe one geological fact, or one set of connexions; and it would also follow in other instances, that such connexions would appear to be implied where they did not exist; merely in consequence of the terms by which rocks, similar in composition, but different in geological characters, were designated. The circumlocutions and difficulties that would follow, in the first of these cases, and the confusion that would result in the latter, are too obvious to be stated.
There is another objection to a mineralogical arrangement, which appears to us of no small importance. It renders of equal value those rocks which are rare, and in some measure accidental, and those which are of the greatest consequence and