Imágenes de páginas

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,
will render its defects much more apparent than they ate in the

Genus 1st. Felspathic.
Sp. 1. Granite, - common granite, with mica only.

2. Protogine, - the same, containing steatite, talc, or chlorite.
3. Pegmatite, - graphic granite.
4. Mimose, - a compound of pyroxene and felspar.

Genus 2d. Amphibolic.
Sp. 1. Syenite, - granite containing hornblende - hornblende

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


Genus 2d. With a base of Mica.
2. Gneiss, - this division contains a very imperfect list of

3. Mica Schist,, one of the enumerated varieties is a gneiss.

Genus 3d. With a base of Schist, (Clay slate.)
Sp. 1. Phyllade, - includes many varieties of argillaceous schist

micaceous, or containing imbedded minerals, and even bituminous marl slate it appears also to contain a variety of gneiss, and some

graywackés. 2. Calschiste, - a mixture of clay slate and carbonat of lime.

Genus 4th. Base of Talc. sp. 1. Steaschiste, - includes talc slate and chlorite slate, together

with many other compound substanees.

Genus 5th. Base of Serpentine. Sp. 1. Ophiolite, - serpentines which contain imbedded minerals. .

We cannot help remarking, that as well in this case as that of the phyllade, we have a striking example of the great inconvenience of a system which separates the simple from the compound rocks; and from a circumstance so, unimportant, in the case of serpentine, as its occasionally containing chromat of iron, or garnets.

Genus 6th. Base of Carbonat of Lime. Sp. 1. Cipolino, - limestone containing mica.

2. Oficalce, - limestone containing serpentine, &c.

3. Calciphyre, – limestone containing various imbedded minerals. We cannot see that any of these incidental varieties have a claim to the title of species; nor is the division even consistent with itself; as the presence of garnet, hornblende, or augit, might as well confer on the varieties of the 3d Sp. the rank of separate species,

Genus 7. Base of Cornénenne. (this is very indefinite.) Sp. 1. Variolite, - certain amygdaloids. 2. Vakite, - other amygdaloids.

Genus 8. Base of Amphibole. Sp. ). Amphibolitę,- a sweeping term, which comprises many differ

ent rocks, in which either hornblende or

actinolite enter as ingredients. 2. Basanite, - this also appears intended to comprise every

rock which has a base of basalt, 3. Trappite, - roches de trapp—we quote the author in the

original, as we can form no definite idea of this species ; and as little, we may add, af the former.

4. Melaphyre, - certain varieties of dark-coloured porphyry.

Genus 9. Base of Amphibolic Petrosilex. This base is not very intelligible—is it a basalt or a dark clinkstone ? Sp. 1. Porphyry, - the last species has a base of amphibole pe

trosiliceux,' and in these varieties the base is ' petrosilex amphiboleux;'-a distinction

too refined for our state of information. 2. Ophite, - green porphyry-surely no more than a variety

of the last species. 3. Amygdaloide,-including the variolites of Durance, and the

orbicular porphyry of Corsica—it also com-
prises some more porphyries, we do not see

4. Euphotide, - Verde di Corsica.

Genus 10. Base of Petrosilex, or of Granular Felspar.
We do not comprehend how these two substances can mean the
same thing.
Sp. 1. Eurite, - including whitestone (weiss-stein), clinkstone,

some clinkstone porphyries (again), and floetz

trap porphyries--a very strange association. 2. Leptinite, - more weiss-stein—and apparently some gra

nites-(hornfels.) 3. Trachyte, - more porphyries with base of petrosilex. We confess that all this appears to us very disorderly.

Genus 11. Base of Argilolite. Sp. 1. Argilophyre,- claystone porphyry. 2. Domite, - claystone with mica.

Genus 12. Base of Pitchstone or Obsidian. Surely so excellent a mineralogist does not mean to confound these two substances. Sp. 1. Stigmite, - pitchstone or obsidian porphyry.

Genus 13. Base indeterminate. Sp. l. Lava, - lavas and scoriæ simple and compound a very

short process for disposing of the volcanic rocks.


Genus 1. Cemented. Sp. 1. Psummite, - appears to contain quartz rocks, micaceous sand

stones, graywackés, and graywacké schists ; and is evidently, even in a mineralogical view, very injudiciously contrived.

« AnteriorContinuar »