« AnteriorContinuar »
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.
source they are drawn; provided they do not trespass on established associations, nor interfere with the leading objects of the nomenclature. As, in the study of natural objects, it is necessary to combine accuracy in the details with comprehensive general views, so, in any system of nomenclature, this leading and important object should be kept in sight. The minutiæ of arrangement, and the trivial details of a highly refined nomenclature, are often injurious by diverting the attention from the greater and more important relations of the objects under consideration. It may sometimes even follow, that analogies which are only apparent, and dependent on the construction of the catalogue, or the nature of the names, may be transferred to the more important positions of the substances, and thus convey prejudices or false views relating to the structure of the globe.
To render a geological system of arrangement complete, its advocates should be allowed the privilege which Brongniart has in his Essay assumed; namely, that of framing terms adapted to the wants of their system. More than this indeed is perhaps required; as, to the existing imperfect nomenclature may easily be traced many of the defects which appear, on a superficial view, to result from the arrangement. As, in the revolutions of Chemistry, it has been found necessary repeatedly to reform the nomenclature; so, in the progress of Geology, it may hereafter be found equally requisite to make important changes in the nomenclature of rocks. The present nomenclature originated in a period of ignorance, and it has been but partially modified through one of comparative knowledge. Rocks have been named, sometimes from their structure, sometimes from their composition, sometimes from their geological positions : while many are still denoted by ancient and unmeaning terms, which are not perhaps the worst with which the catalogue is deformed. To adopt terms derived from so many sources, and to preserve the consistency of a catalogue or an arrangement, is impossible; nor is it easy to make a partial selection, or useful alterations, without great inconveniences. In the present state of the science, it would be a rash experiment to reform the nomenclature altogether, as the science is not ready for such a reform. To supersede the use of terms long associated with all our ideas, is at all times a proceeding which nothing can justify but the most decided advantages, and the most absolute certainty that we are proceeding on a correct basis. We consider it far better to submit to the defects as they now stand, than to incur the risk of others, certainly far worse; and would much rather endure both repetitions and circumlocutions, than encounter the
confusion which invariably results from the ambiguous use, and the frequent changes of terms. The attempts of the advocates, of a Mineralogical classification to introduce new terms, have not been attended with success; although less productive of in- . convenience, and flowing from high authorities. To make such an attempt in a geological system of arrangement would demand both authority and advantages proportioned to the greater inconveniences by which it would be attended.
In examining the present nomenclature for the purpose of seeing more distinctly in what manner it interferes with the con-, sistency of a geological arrangement, it will immediately be seen, that the most prominent fault is the adoption of a double principle of nomenclature. Rocks are thus, as we already remarked, named, sometimes from their nature or their composition, --sometimes from their position, or their geological character; while that inconvenience is increased in many instances by the capricious mode in which either of those principles is adopted. An example will illustrate our meaning. There is often no difference between the argillaceous schists of the primary, and those of the secondary strata; and there is often a perfect resemblance between certain granitic compounds, occurring in the primary rocks, and in the traps of the latest origin. But in the first case, from difference of position merely, these rocks are called respectively clay slate and slate clay, or shale; while the common term, greenstone or syenite, is applied to two rocks, differing most widely in their geological positions. We need scarcely here notice the greater confusion arising from an application of the term greenstone to stratified and to unstratified rocks; as this is rather one of the collateral evils which arise from neglect, from systems or from ignorance. It appears most important to preserve consistency in this respect: For otherwise this practice may be made to serve the purpose of almost any hypothesis. The relative position and geological nature of a rock may thus be determined from its mineral composition; and that again from its geological position, and the system made quite smooth and easy by a vicious reasoning in a circle. To enumerate the cases where this convenient process has been adopted, would be to extend these remarks beyond the space we can spare for them; but geologists will be at no loss to recal them to their recollection. It is time indeed to draw to a close; and in so doing we shall barely observe that, in the present state of things, there seems no remedy for the evils arising out of this ambiguity, but that of accompanying any geological arrangement of rocks that may hereafter be adopted, by adequate definitions, or explanations of their geological connexions, and
Fourt, we shallo e preferring tinute, an
of the views of the author respecting the places which they occupy in the structure of the earth, and the analogies by which they are mutually related.
Having thus stated the arguments and objections that seem of chief importance in this dispute, and, as we trust, shown sufficient cause for preferring the geological method of arrangement, we shall give a brief sketch of the classifications of the four authors in the Essay under review. The disadvantages of a mineralogical arrangement, for the purposes of geological science, will thus become practically apparent on the one hand; although, on the other, it will be seen that the two last authors, treading in the antiquated steps of their master, instead of following the path of Nature, have left us nothing but the shadow of a hypothetical classification.
T'he superiority of Brongniart's work, no less than the reputation of its author, induces us to give his classification complete, but in the briefest abstract which we can make. The others must be passed over more hastily. We have not room to indulge in many remarks, nor will they be necessary to the geological reader: a few will suffice to point out the places of the more prominent defects. We shall translate the foreign terms that may be required into the synonimes most in use; but we have too little confidence in the eventual adoption of the author's neology, to think it necessary to give an English physiognomy to the Gallicized Greek compounds in which he deals.
The brief form into which this arrangement is here condensed,
Genus 1st. Felspathic.
2. Protogine, - the same, containing steatite, talc, or chlorite.
Genus 2d. Amphibolic.
schist containing felspar, &c. 2. Diabase, - greenstone-hornblende schist containing fel.
spar-greenstone porphyry-orbicular gran
ite of Corsica. 3. Hemithrene,- a hornblende rock containing carbonat of lime. Class II. CRYSTALLIZED Rocks. (AniSomeres.)
Genus 1st. With a base of Hyaline Quartz. Sp. 1. Hyalomicte, - quartz and mica--probably a variety of quartz