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interest, as far as the structure of the earth, or the natural history of these substances, is concerned. The most rare modifications, the most limited varieties, would thus claim as much attention as those which are the most constant and the most cominon; while differences, which may be of the greatest importance in geological science, but would not excite much attention from their mineralogical composition, may thus pass with much less notice than they demand. Hornblende schist will furnish the reader with an easy illustration of this remark. In the same Geological connexions it may be simple, or it may contain a few particles of felspar; but, according to a Mineralogical system, it must be designated by two distinct terms; and, what is worse, the simple rock will not even be found in the arrangement at all, but must be sought for among the simple substances in the system of mineralogy.
If, therefore, in one sense, a mineralogical arrangement of rocks is natural, it is, in a much more important view, unnatural, or artificial; as it disjoins the wider and more interesting affinities by which these substances are connected with each other and with the general structure of the earth. It is, in fact, an artificial system, with the imposing appearance of a natural one; founded on a minute set of appearances, and negligent of the larger features, and the numerous important circumstances of affinity, or difference, which prevail among the objects of its contemplation. Thus it in some measure resembles the artificial arrangements of the ancient botanists as compared with the more philosophical views of the moderns in their establishment of Natural Orders. Mineralogists, indeed, appear in this instance to have been misled by the example of Linnæus, and by the valuable consequences that have resulted from his systems in the organized departments of natural history; forgetful of the important and radical differences by which these departments are distinguished from the peculiar objects of their study.
Were it possible to make any arrangement, however artificial, which should facilitate the study of rocks as constituent parts of the earth's structure, it would form a valuable acquisition to the geologist, as well as to the collector of specimens. But if, in teaching the latter to arrange his cabinet, it misleads the former-it is injurious and not beneficial. In the present state of our knowledge, it appears indeed a vain attempt, as well as an inconvenient and injurious sacrifice to the formalities of an imaginary logic.
In proceeding to consider the comparative advantages and defects of a Geological arrangement of Rocks, we think that the authors under review have not been sufficiently careful in dis
tinguishing between the study of minerals and that of the structure of the earth. The connexion of the former with the latter is doubtless an important part of their history; but the knowledge of minerals can never form a proper basis for the arrangement of the great masses of which our globe is composed. If our sole object in the study of rocks were a knowledge of their mineralogical composition, such an arrangement would doubtless be the best. But the main end of that study is to investigate their proportions, their gradations, their analogies, their mutual dependence or connexion, their order of succession and disposition--in short, their general relations of all kinds, to each other and to that structure which forms the object and business of geology: And this, as it appears to us, can only be attained by a classification founded on a geological basis. Such a classification has, for its foundation, the most extensive affinities and the most important characters of the objects to be arranged; and it thus in some measure resembles a classification of plants, according to their natural orders. While it instructs us in the history of rocks as constituent parts of the earth, it does not exclude their history as mineral compounds: since we are enabled to combine with the former the most minute subdivisions of varieties; and at the same time have it in our power to separate the accidental and unimportant from the constant and essential. It also appears to us that it affords equal facility for reference as a mineralogical arrangement, by the very simple expedient of brief synoptic tables; thus combining their greater and their lesser analogies; their order in nature with their mineralogical affinities.
It cannot however be denied, that the objections to such a system of arrangement are both numerous and weighty; and it is only by comparing these with the advantages now stated that we can be guided in our choice.
The most formidable objection is the imperfect knowledge which we at present possess of the true order of rocks in nature. Whatever system therefore we adopt as the basis of such an arrangement, must confessedly be imperfect. But it may still be such as to be capable of perfection; and it offers a basis not only susceptible of correction, but gradually increasing in correctness; since every step adds something to the mass of facts, on which it is founded.
It must also be admitted, that a geological arrangement cannot be logically correct; since it cannot be founded on one simple and consistent principle. While the larger divisions are derived from the general order which rocks hold in nature, the smaller are necessarily founded on mineralogical characterse
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Hence also follows this very obvious inconvenience, or rather irregularity, namely, that, as the latter are subject to the forma er, the same mineral compound may occur, as it in fact sometimes does, in more than one of the larger or geological divisions. This defect appears at present irremediable; but, such as it is, it must be examined, like the system itself, not on logi. cal principles, but on the principle of utility.
In the other departments of nature, the objects are, in genes ral, definite, constant, and connected by simple and invariable relations. A rigid adherence to an adopted system of arrange ment thus becomes as useful as it is easy. It is the utility, in fact, rather than the consistency of any such system, which constitutes its merit, and in transferring to another class of objects those rules to which, from their nature, they are not a. menable, we mistake the end for the means. This is to be anxious about words, and negligent of things. An arrangement of rocks ought in fact to be considered as a branch of geological science, and a history of their natural affinities, as far as that is practicable. To the elucidation of that science, all minor considerations ought to be rendered subservient; even at the risk of some inconsistencies of order, or the sacrifice of logical forms. Our first object should be to select that order of arrangement which is most useful: if an unexceptionable regularity could be superadded to utility, such a system would be perfect; but a precision which tends to no useful purpose is za mere piece of pedantry and delusion.
We are by no means inclined to suppress the objections to a geological arrangement; on the contrary we are anxious to point them out, as they must be known before they can be remedied: and we have therefore studied to add to those which Brongniart has suggested. The number and value of these ob. jections will perhaps be rendered most apparent by examining the conditions required for a perfect geological arrangement, and by noting where these are defective.
In the first place, the order of every rock in nature ought to be known; and, to render such a system of arrangement perfect, it ought also to be constant. Not only should every rock be constant in its geological relations, but its mineral characters should be definite and invariable. Further, it would be requisite that under every principal substance, whether it be called genus or species, a distinct set of varieties should be found, and under those only.
But, unfortunately, no constant and definite order of succession among rocks has yet been discovered : and it is indeed now certain, that no order can be assigned which is not subject to nu
merous exceptions, both in the larger features, and in the minor details. Besides this, individual rocks are subject to frequent changes of their mineralogical characters; often passing into each other by imperceptible gradations: an objection, however, which was already noticed as militating against a mineralogical arrangement. It is, lastly, a cause of great inconvenience, that certain rocks, resembling each other in composition, are somes times found in situations far remote in geological connexion.
We might have dwelt in greater detail on these defects, and illustrated them by examples, but our limits do not admit of it; while to the geological reader, for whom alone such details could have any interest, it can scarcely be thought necessary. However serious they may be considered, and however they may detract from the regularity or perfection of a geological arrangement, they do not destroy its utility. Many of the defects admit of a remedy, by adopting some repetitions, and by making some small sacrifices to order: trivial inconveniences, which still leave the classification in a great degree equally useful for practical purposes.
Having thus acknowledged the defects of a Geological classi.. fication of rocks, as they have struck us, it is a justice due to Brongniart, and to others who are the advocates of a Minera- . logical arrangement, to state their objections also; to most, if not all of which, we think we can make satisfactory replies, if indeed some of them have not already been anticipated in the preceding remarks.
It is considered an objection, that the simple and compound rocks are included in the same arrangement; the description of the former being superfluous, as they have already been found in the mineralogical system. But we, on the contrary, consider this as an advantage; as the geological relations of these rocks are frequently the same, and as they often pass imperceptibly into each other. Even admitting the propriety of describing the simple rocks in a system of mineralogy, great inconvenience must follow from omitting them in a classification of rocks; from causes too obvious to require mention.
In some instances in nature, the same rock occurs in two distinct geological positions, as we have already noticed; and it is therefore considered as an objection that it would appear in two places in a geological arrangement. The inconvenience, such as it is, appears to us very trifling, and indeed admits of an easy remedy by some method of reference. But we even consider the arrangement as advantageous in this case; since it is an important part of the geological history of a rock, to know that it occurs under different positions and in different associations.
It is further objected, that a geological arrangement is hypothetical and difficult of application. Every day diminishes the validity of this objection, and it will cease altogether whenever the science shall be perfected. But it may be retorted also, on the other hand, that if the rival system is free from hypothesis, it is only because it labours under the much greater defect of excluding the natural affinities of rocks, and is thus nearly useless for the purposes of science.
The last objection which appears to possess any weight, is, that different mineral compounds are sometimes enumerated under one name, and the same compounds under different names. This certainly is an evil by no means irremediable; but it is a question how far, in the present state of the science, it admits of à remedy without introducing still greater inconveniences. It involves the difficult question of a nomenclature: a difficulty from which the mineralogical method is not exempt, and which, if we are to judge from Brongniart's attempt, it has by no means overcome. In a nomenclature merely mineralogical, the multiplication of names could produce no great inconvenience beyond that arising from their numbers. As in mineralogy, it would merely serve to regulate and describe a cabinet of specimens. A nomenclature founded on mineralogical characters, is indeed perhaps necessarily mi. nute; but the numerous combinations of minerals, and the end. less varieties of aspect thus presented, render it impossible to apply distinct names to all. So that, even in this respect, a mineralogical arrangement is almost unavoidably imperfect; to say nothing of the new terms which would be required to render it even tolerably complete, and which are always productive of inconvenience.
But as the study of rocks, according to the view which we have taken of this question, is principally required for the purposes of Geology, so, it appears to us, the nomenclature should as far as possible be rendered subservient to that end. It is in the first place obvious, that, for the purposes of geological description, general terms are absolutely required. Otherwise, as numerous substances occur under one general relation, unavoidable confusion, as well as tedious details, would be the inevitable consequence. These general terms should also be founded on the geological relations; or should be such at least as are likely, from their former application, to convey a true notion of the positions and analogies of the rocks in question. With regard to the inferior terms required for the details of varieties or inferior divisions of any kind, it seems indifferent from what
VOL. XXXIV. NO. 67.