I believe I have stated fairly the theory proposed by Monsr. Menard de Groye, to explain the distinction between basalt and lava, and have enabled my readers to judge, how far my ideas have been anticipated by this author; for, although I did not read his memoir, until after my treatise was composed, he has of course a fair claim to priority, where the opinions are the same. It will be seen however, that though Menard finds himself obliged (as I conceive every geologist will be, who examines with attention the Tertiary volcanic rocks) to make a distinct class of them, yet he does not explain the cause of their differences on the same principles as myself. According to him, the differences arise from their being formed partly above, partly below the surface of water; whereas my hypothesis assumes them to arise from their being formed under a body of water less considerable than the ocean. With respect to basalt his views are diametrically opposite to mine, as he considers it to be so formed in consequence of the absence of water, whereas I have explained its compactness from its being produced underneath that fluid. Indeed Monsr. Menard does not appear in any part of his memoir to allude to the difference in the state of compression produced by the presence or absence of water. Dr. Boué in his memoir on Germany, has also alluded to a similar distinction (Journ. de Phys. Vol. 95) of volcanic products, which he divides into those caused by volcanos burning in the open air, and by the same more or less submarine, or burning under water. Under the head of those partially submarine he includes trachytes and many basalts, and this division obviously corresponds with my second class of Tertiary volcanic products. The rocks included by Dr. Boué and by myself are nearly the same. Dr. Boué has also noticed in common with myself the following distinctions between submarine and subaerial volcanos: viz. that they originate from dykes, form mountains of inferior height, and are associated with tuffs possessing a strong degree of aggregation. He also remarks the greater frequency of crystalline infiltrations, and the more decided changes effected upon the surface of the ambiguous rocks. It is satisfactory to find my observations thus confirmed, or rather anticipated, as it proves that they have not been imagined for the sake of propping up an hypo thesis, but that the principles laid down by Sir J. Hall admit of a more extended application, than appears, so far as we can collect from his writings, to have been anticipated by their author. Note to Page 429. * That I am warranted in speaking as I have done of the oscilla tions of opinion which have prevailed respecting the origin of basalt, will be evident from the following passage in Daubuisson's account of the basalts of Saxony, translated by Neill. "It appears, says the celebrated chemist of Berlin, that naturalists are recovering by degrees from the volcanic illusion. It is about fifty years since a French naturalist revived the opinion concerning the volcanic origin of basalt, and he lived to see almost all Europe adopt his sentiments. Bergman, the first of the chemists who employed himself with diligence and success in examining mineral substances, and who, to an intimate acquaintance with the effects of heat, joined an extensive knowledge of mineralogy, could not bring himself to consider basalt as a product of volcanic eruptions. The Swedes adopted his view of the question. It is scarce forty years since every body in Germany considered basaltic mountains as antient volcanos. Werner lifted the Neptunian ́standard; and now, among all the German mineralogists of any reputation, I know but one (Voigt) who still retains the old doctrine. We have already seen in how decisive a manner Klaproth has pronounced on the subject: he, of all the German chemists, has had most opportunities of observing the effects of fire on mineral substances, and he has besides studied the history of basaltic mountains with that correctness for which he is remarkable. In Ireland, Mr. Kirwan was a supporter of the volcanic doctrine; but the numerous chemical experiments which he made on minerals, and other considerations, led him to a change. Dr. Mitchell, one of the very best mineralogists, and Mr. Jameson, the author of the Mineralogical Travels in Scotland, and the greater part of British naturalists, consider basalt as having been produced in the humid way. The geologist who of all others possessed the greatest experi* Klaproth, Journ. des Mines, No. 74. ence, Saussure, the illustrious mineralogist of the Alpsfound it necessary, in the latter part of his life, greatly to limit his notions as to basalt being of volcanic origin. In speaking of the extinguished volcanos of Brisgau, he says, "I acknowledge, that before studying the writings of Werner, I felt no hesitation; but that philosopher has taught me to doubt." Dolomieu, who was at the head of the Vulcanists, but in whom the love of truth was paramount to the spirit of party, admitted that some basalts had been produced in the humid way. Ile observes, "I have circumscribed the volcanic empire more than any other mineralogist, French, English, or Italian, having withdrawn from its dominion many mineral substances formerly placed under it. I hold that the basalts of Saxony, of Scotland, and of Sweden, may claim Neptunian origin." When treating of the basalt of Ethiopia, he adds, "I may affirm with certainty, that it is not of volcanic production."+ I shall not extract the remarks that follow, in which Daubuisson, not very logically, I think, anticipates the complete overthrow of the volcanic theory, from the change that he represents as having taken place in the opinions of naturalists on this subject. I confess, I should have rather have been disposed to argue from the foregoing statements, that the question had not been sufficiently sifted; for, on speculative subjects at least, such oscillations of opinion as are described show a defect of evidence, even more than the influence of authority. If the former did not exist, it is hardly possible that the latter should prevail, ou a question of pure science, to such a degree as to overpower reason. Experience has since shewn such to have been the case, for the preponderance of opinion at present in favour of the volcanic theory is fully as great as it ever was in opposition to it, even when Werner's reputation was at its height. It is for the Geologists of the present day to take care, that they also have not been misled by the influence of great names, and that the evidence on which they proceed be such, as to afford a reasonable security against any similar change of opinion. These considerations rendered me cautious in making up my Journal de Physique, tom 37. + Daubuisson on Basalt. Neill's Transl. P. 163. mind with regard to the universal igneous origin of basalt, even after visiting Auvergne; and at present they lead me to regard a detailed examination of every basaltic country a necessary preliminary to a decision with respect to its origin. It is only thus that we can proceed on sure grounds, and ought to flatter ourselves that we are laying a stable foundation for a future theory of the earth. I amused myself some years ago with drawing up a Table, intended to represent the gradation of opinion on subjects of this nature, entertained at that period by numerous Geologists, and though I am aware, that it does not express in many instances exactly the notions of these individuals at the present time, any more than it does of myself, yet, as it has already found its way into a Journal, it cannot be improper to give it a place in this volume. |