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Show 58 WERNER; by several geologists in Italy and elsewhere ; and h.is leading di~ visions of the secondary strata were at the same time made the basis of an arrangement of the British strata by our coun. tryman, William Smith, to whose work we shall return byand- by. In regard to basalt and other igneous rocks, W erner's theory was original, but it was also extremely erroneous, The basalts of Saxony and Hesse, to which his observations were chiefly confined, consisted of tabular masses capping the hills and not connected with the levels of existing valleys, like man~ in Auvergne and the. Vi:varais. These. basalts, and all other rocks of the same family m other countries, were, according to him, chemical precipitates from water. He denied that they were the products of submarine volcanos, and even taught that, in the primeval ages of the world, there were no volcanos. His theory was opposed, in a two-fold sense, to the doctrine of uniformity in the course of nature ; for not only did he introduce, without scruple, many imaginary causes supposed to have once effected great revolutions in the earth, and then to have become extinct, but new ones also were feigned to have come into play in modern times; and, above all, that most violent instrument of change, the agency of subterranean fire. So early as 1768, before Werner had commenced his mineralogical studies, Raspe had truly characterized the basalts of Hesse as of igneous origin. Arduino, as we have already seen, had pointed out numerous varieties of trap-rock in the Vicentin, as analogous to volcanic products, and as distinctly referrible to ancient submarine eruptions. Desmarest, as we stated, had, in company with Fortis, examined the Vicentin in 1766, and confirmed Arduino's views. In 177~, Banks, Solander, and Troil, compared the columnar basalt of Hecla with that of the Heb· rides. Collini, in 1774, recognised the true nature of the igneous rocks on the Rhine, between Andernach and Bonn. In 1775, Guettard visited the Vivarais, and established the relation of basaltic currents to lavas. Lastly, in 1779, ~..,aujas published his description of the volcanos of the Vi varais and V elay, and shewed how the streams of basalt had poured out from craters which still remain in a perfect state*. When sound opinions had for twenty years prevailed in Europe concerning the true nature of the ancient trap-rocks, "' Cuvier, Eloge de Desmarest. DESMAREST'S MAP OF AUVERGNE. 59 Werner by his dictum caused a retrograde movement, and not only overturned the true theory, but substituted for it one of the most unphilos?phical ever advanced in any science. The continued ascendancy of his dogmas on this subject was the more astonishing, because a variety of new and striking facts were daily accumulated in favour of the correct opinions first established. Desmarest, after a careful examination of Auvergne, pointed out first the most recent volcanos which had their craters still entire, and their streams of lava conforming to the level of the present river-courses. He then s~ewed that there were others of an intermediate epoch, whose craters were nearly effaced, and whose lavas were less intimately connected with the present valleys ; and, lastly, that there were volcanic rocks still more ancient, without any discernible craters or scorire, and bearing the closest analogy to rocks in other parts of Europe, the igneous origin of which was denied by the school of Frey berg *. Desmarest's map of Auvergne was a work of uncommon merit. He first made a trigonometrical survey of the district, and delineated its physical geography with minute accuracy and admirable graphic power. He contrived, at the same time, to express, without the aid of colours, a vast quantity of geological detail, the different ages, and sometimes even the structure of the volcanic rocks, distinguishing them from the fresh-water and the granitic. They alone who have carefully studied Auvergne, and traced the different lava streams from their craters to their termination,-the various isolated basaltic cappings,-the relation of some lavas to the present valleys,the absence of such relations in others,-can appreciate the extraordinary fidelity of this elaborate work. No other district of equal dimensions in Europe exhibits, perhaps, so beautiful and varied a series of phenomena; and, fortunately, Desmarest possessed at once the mathematical know ledge required for the construction of a map, skill in mineralogy, and a power of original generalization. Dolomieu, another of Werner's contemporaries, had found prismatic basalts among the ancient lavas of Etna, and in 1784 • Journ. de Phys., vol. xiii. p. 115; and Mem. de l'lnst., Sciences, Math~mat. et Phys., vol. vi. p. 219. |