Imágenes de página
PDF
ePub

not essentially differ from those that may be observed in other basaltic districts. The frequent occurrence of mining galleries, however, very considerably enhances the value of M. Daubuisson's researches ; since these excavations occasionally reveal the beds of clay or sand on which the basalt rests, or afford convincing proofs of the gradual transition which connects the lastmentioned rock with wacke. The basalt of the hill of Lichtewalde is remarkable for the profusion of olivine which it contains; and which occurs in amorphous masses that are sometimes larger than a man's fist. In some cases, they indicate a tendency to crystallization, exhibiting the rude semblance of a prism, - with two planes cutting each other at right angles. If we accurately recollect, M. Bort procured, in the Isle of France, specimens of the crystals of olivine, remarkably well defined : but such are rarely to be obtained.

The columnar disposition of the basalt at Stolpen is duly commemorated, and suggests a sentimental reflection which steals on the reader with an agreeable surprize :

• The columns of Stolpen are employed as posts or boundarystones on the roads, and in the streets of the neighbouring towns ; and they are sometimes placed in gardens and pleasure-grounds, as objects of decoration. The basalt is used also in the manufacture of bottles. While walking about the church-yard of Stolpen, I remarked a group of basaltic columns, tastefully placed over a grave. Such a sight could not fail to bring to mind the recent loss of the celebrated geologist of France, (Dolomieu,) who has treated so well of basalt ; and I could not help thinking that a similar simple minera. logical trophy might with propriety be raised over his tomb, as cal. culated to perpetuate the memory of his labours. In former times, a sphere circumscribed by a cylinder was placed over the sepulchre of Archimedes; -a spiral logarithmic curve was inscribed upon the monument of Bernouilli ;- and both emblems were fitted to recall the at. tention of posterity to the discoveries and writings of these eminent men.'

From the circumstance of the Saxon basalts uniformly resting above and never under the fundamental rocks of their respective mountains, the author infers that they are of posterior formation to these rocks; and that they are of a totally distinct character from the granite, mica slate, porphyry, sand-stone, and gravel, on some one of which they are found to repose. When the surface of one of the mountains is inclined, or stratified, the surface of superposition of the basalt is not parallel with the direction of the strata ; whence he concludes that, before the deposition of the basalt, its sustaining beds had undergone considerable changes and disintegrations. As the principal characters and qualities of basalt may be found in other rocks, he

presumes

presumes that it has been formed by similar agency; especially as the fact of its having had a different origin remains to be proved. While granite, porphyry, sbistus, &c. have been regarded as precipitates from a solution which once covered the countries in which they are now found, we may, in like manner, ' consider the basalt of Saxony as a precipitate or sedia ment proceeding from a solution which had at one time stood suspended over the whole of that country.'- In Saxony, basale occurs only on the summits of mountains, and must therefore have been either originally deposited on them exclusively, or the present caps and platforms of that substance must be merely the relics of a great deposition of basalt which, at one time, covered all the face of that region, and M. Daubuisson endeavours to shew, that the latter alternative is supported by existing facts and appearances. The intimate relation of basalt with wacke and clay, and the transition of these substances into one another, seem to prove that the first of them is only an argillaceous mass, of a black colour, impregnated with ferruginous matters,and of which the particles have contracted a strong degree of cohesion. - From the identity of constitution manifested by basalt and green-stone, we are warranted to assert that the for mer is merely a compact green-stone, or hornblend rock.

The proofs adduced in the fourth part against the volcanic origin of the Saxon basalts are, first, the striking dissimilarity between the mountains described, and those in which volcanic agency is known to exist at the present day; 2. the absence of any visible opening, or funnel, of which not a vestige has been traced ; 3. the want of any crater; and 4. the physical impose sibility of the basalt having been projected up through the granite, &c. without producing evident disruptions, and of its making its escape in the line of the greatest resistance. • The supposition, that all the basaltic summits of Saxony are the remnants of one great stream of lava, appears to this author to be equally untenable, from the following considerations ; 1. The supposition itself implies entire ignorance of the geology of this district of country, the bowels of which have been explored, for upwards of six centuries, by numbers of miners and mineralogists. 2. In the whole range of this mountainchain, no scoriz, cinders, nor scorched stones, are to be found. 3. Every basaltic summit, or platform, presents a continuous mass, which indicates a simultaneous formation, inconsistent with all the operations of volcanoes with which we are acquainted; and 4. The very formation of basaltic beds is alto

irreconcileable with the extravagant notion of one great and general eruption. D 3

Other

Other arguments against the volcanic origin of basalt are next urged from the nature of that substance in general, as from its passage into wacke and clay ; its being only a modification of green-stone, which is composed of ingredients readily fusible at a moderate temperature; the sameness of its principal characters all over the world, while lavas present very striking differences ; the similarity of specimens from very different repositories; and the large proportion of iron which it invariably contains, and which no other rock appears to be capable of affording. The adventitious substances included in basalt, viz. crystals, grain, balls, geodes, fragments of old rocks, and water, are also quoted as proofs of its production in the humid way. This part of the reasoning is conducted in the. usual style of the Wernerians; and, indeed, these points have been so frequently agitated in almost every geological publication, that it would now be idle to state or to confute them. Considerable stress is likewise laid on the regularity of the beds of basalt, their disposition to a columnar arrangement, and especially their alternating with strata of bituminous and combustible substances, as appearances altogether incompatible with their igneous origin.

M. Daubuisson scarcely deigns to notice the supposed vestiges of craters in the basaltic hills of Saxony, because he perceived no excavations which were not the effect of human exertion, or of some ordinary cause, easily detected. The concluding paragraph of the fourth part bespeaks at least moderation and candour :

• Such are the principal reasons which have convinced me that the basaltic rocks, which I have hitherto had an opportunity of seeing, have not resulted from subterranean fires. - My purpose here has not been to treat fully the question concerning the origin of basalt in general : I have not advanced any thing not founded on my personal obseryations ; and I have only deduced such conclusions from them as ap. peared to be inevitable. Perhaps at some future day, after I myself may have viewed volcanoes and their products, and examined the basalts and extinguished volcanoes of Auvergne and the Vivarais,

- perhaps I may then be better fitted to consider that question in all its extent, to form a just estimate of what has been written on the subject, and to bring forward more interesting facts and examples. One of my fondest wishes is to undertake such a journey, after com. pleting the preparatory examinations which now engage me. There can scarcely be more interesting problems to French naturalists, than to determine if so great a portion of their country has really at one time been the theatre of the magnificent and awful scenes presented by volcanoes; if the soil which is now cultivated has, at some remote period, been an ocean of fiery matter. - We are now aware of the kind of facts which must be resorted to in solving such a problem.'

The

The translator, in a note, acknowleges that his author accomplished his projected visit to Auvergne ; and that he now concurs with Dolomieu, and other naturalists, in regarding the puys, or conical eminences of that country, as extinct volcanoes, and the circumjacent basalt as of volcanic origin, or basaltic lava : without, however, relinquishing his opinion concerning the aqueous origin of the basalt of Saxony.

Ascending, in the fifth part of his treatise, to more general conclusions, M. Daubuisson deems it fair to hazárd, with the diffidence of philosophical conjecture, the position that, as all the basalts of which he has read descriptions, or examined cabinetspecimens, agree in their leading characters, and as the origin of those which he has explored in Saxony cannot be attributed to fire, the others have not been produced in a different or opposite manner. While he admits, however, the possibility of the existence of basaltic lavas, he understands, by this last mentioned expression, beds of basalt that have been fused by the combustion of beds of coal immediately beneath them : - but he contends that appearances in this case must be very limited, when compared with the extensive and alternating ranges which he describes. The contrary opinion he refers to a love of the marvellous, and the fondness with which the imagination contemplates the grand and striking phænomena of its own creation. Hence, in the language of Klaproth, naturalists are gradually recovering from their volcanic illusions. In the course of half a century, almost all the German geologists have aban, doned the doctrine which identifies basalt with lava ; and even Saussure and Dolomieu felt themselves constrained to modify and circumscribe this tenet of their creed.

The concluding exposition of the nature of basalt offers to our notice little that can be regarded as original. The attempt to explain the amygdaloidal structure of some of the varieties is, in particular, far from satisfactory; and we cannot, by any means, acquiesce in the notion of the prismatic arrangement of this substance being the result of contraction, in the course of drying and hardening. A more rational hypothesis is that which rests on the doctrine of crystallization.

We close our report by remarking that this French geologist is a powerful pleader for the cause which he advocates; that his arguments derive their importance chiefly from the matters of fact which his opportunities enabled him to contemplate; and that, although the text and copious notes afford a clear and perspicuous exposition of the Wernerian views relative to the formation of basalt, the subject, when examined in all its bearings and details, and when treated as a problem of general application, is not unattended with serious and, perhaps, in. superable difficulties.

ART.

p4

ART. VI. Philosophical Transactions of the Royal Society of

London. for the Year 1814. Parts I. and II.
[Article concluded from the Review for December, p.426.]

CHEMICAL and PhysIOLOGICAL PAPERS, Part II.
OBSER
BSERVATIONS on the Functions of the Brain By Sir Eve-

rard Home, Bart. — Having remarked on the little success which has attended the attempts hitherto made to ascertain the functions of the individual portions of the human brain, Sir Everard here proposes, in order to throw some light on this subject, that those who have an opportunity of witnessing.injuries of the brain, and the consequent change in its functions, should publish the result of their experience in one connected view. He has accordingly adopted this plan in the paper before us; stating the general facts and conclusions in the text, and placing in the form of notes the more minute particulars. He arranges his materials in 10 sections; ist. The effects produced by an undue pressure of water on the brain ; 2d. The effects resulting from concussion of the brain; 3d. The effects which ensue when the blood vessels of the brain are preternaturally dilated or diseased ; 4th. The effects produced by extravasated blood; 5th. The effects arising from the formation of pus; 6th. The effects of depression, and the thickening of different portions of the skull; 7th. The effects of pressure from tumours; 8th. Effects of injury to the substance of the brain ; gth. Effects of alteration of structure in the brain; 10th. Effects of injury to the medulla spinalis. The number of facts brought forwards is so very considerable, as to render this paper a valuable repository of information; and we are well aware how much more important this is than the most ingenious theory: yet we think that Sir Ev. Home might with advantage have embodied his experience in the form of some general deductions, which, if cautiously formed, would have greatly enhanced the value of his labours.

Further Experiments and Observations on Iodine. By Sir H. Davy, LL.D. &c.-In our last No., P: 413., we gave an ac, count of Sir H. Davy's experiments on the newly discovered substance to which the name of iodine has been assigned. The present paper, which is dated from Florence, supplies a continuation of the labours of this distinguished chemist. It is arranged under five heads; the first of which is intitled, on the triple compounds containing iodine and oxygen.' As a specimen of this description of bodies, Sir H. details the properties of the compound formed with potassium :

· The triple compound of potassium purified by alcohol is almost tasteless, has no action on vegetable colours, is very little soluble in

cold

[ocr errors]
« AnteriorContinuar »