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IV.—The Permian Formation in the Alps of Piémont, Dauphiné, and Savoy
Published online by Cambridge University Press: 01 May 2009
Extract
In a recent paper on the Marble District of the Apuan Alps or Carrara Mountains I showed that the gneissose schists which form the nucleus of that range, and upon which rests the Triassic marmiferous formation, are, not of Archæan, but, upon irrefutable palæontological evidence, of Palæozoic age, and that, upon equally conclusive lithological and stratigraphical evidence, they must be assigned to the later part of that period, that is, to the Lower Permian. The former conclusion was first arrived at in the course of the geological survey of the range by Lotti and Zaccagna and upon the palæontological authority of the late Professor Meneghini; the latter conclusion was chiefly the result of the striking analogy, first pointed out by Zaccagna, between the stratigraphical sequence and lithological characteristics of the Apuan Alps and the Montgioie range of the Maritime Alps which forms the divide between Southern Piémont and the Western Italian Riviera.
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References
page 7 note 3 Geol. Mag., December, 1915, pp. 554–65.Google Scholar
page 8 note 1 These surveys, accomplished in four consecutive short summer seasons, and embracing the Maritime and the whole of the Western Alps, including the Mont Blanc massif on both sides, constituted on Zaccagna's part a veritable tour de force, enhanced by his exhaustive reports in the Bollettino R. Com. Geol. d'Italia of 1887, pp. 346–417, and 1892, pp. 173–244 and 311–404, with maps and sections. He also compiled, with Issel and Mazzuoli, an excellent geological map 1 : 200,000 of the Ligurian and Maritime Alps for the Italian Alpine Club in 1887. The circular examination of the Mont Blanc massif was carried out by Zaccagna himself, while a section from the Arve (Chamounix) Valley across Mont Blanc by the Col du Géant to the Dora Baltea (Aosta) Valley was taken by Mattirolo.
page 9 note 1 “Fossili Paleozoici Alpi Marittime”: Acad, Atti R.. Lincei, 1877.Google Scholar
page 12 note 1 Portis, A., Boll., R. Com. Geol., vol. xviii, p. 417, 1887; Mazzuoli, L.,Google Scholaribid., p. 6; S. Squinabol, Giornale Scient. Genova, Fascic. Giugno, 1887. The survey of the Ligurian Alps eastward from the Montgioie range, surveyed by Zaccagna, was carried out concordantly by Mazzuoli and Issel, Boll. R. Com. Geol. 1884 et seq.
page 13 note 1 The Mont Genèvre group and Mont Chaberton have been dealt with at length in the interesting papers respectively by Cole, and Gregory, , Q.J.G.S., 1890, p. 305 et seq., and by Davies, & Gregory,Google Scholaribid. 1894, p. 307 et seq.
page 13 note 2 Near Moutiers are the two geologically famous localities of Petit Cœur and Mont Jovet in the Tarantaise district of the Isère Valley. Near Petit Cœur, about 6 kilometres north of Moutiers, the long-debated phenomenon of a Carboniferous, fossiliferous stratum being wedged between two strata of Jurassic fossiliferous limestone was interpreted, among others by Lory, as due to a fault, whereas Zaccagna explained the Carboniferous strip more naturally as the remnant or denuded extremity of a synclinal fold, the other end of which appears in a somewhat larger outcrop at Hautecour, some 6 kilometres east of Moutiers. In Mont Jovet (2,303 metres), on the other hand, the puzzling feature was its being capped by a considerable mass of calc-schist with pietra verde or vert des Alpes intercalations, surrounded by a Triassic belt. The former outcrop was regarded by Bertrand as Liassic, while Professor Lory included it in his Triassic metamorphosed schistes lustrés, and lastly, Zaccagna recognized it as schistes lustrés but of Archæan age, the pietra verde intercalations being conclusive evidence by analogy with Mont Genèvre, Susa, etc. Professor Gregory, in his searching analysis of all the evidence (Q.J.G.S., 1896, pp. 1–16), concludes in favour of the pre-Carboniferous age of the Mont Jovet schists, viz. in the absence of the Lower Palæozoic, virtually in favour of their Archæan age.
page 14 note 1 The Val Veni depression between these two mountains and the granite massif of Mont Blanc is filled with Liassic limestone resting conformably against, the Permian of the former but unconformably against the latter.
page 14 note 2 “Étude sur la constitution géologique du massif de la Vanoise”: Bull. Carte géol. France, vol. ii, No. 20, 1891.Google Scholar
page 14 note 3 A sketch-section of part of the Mont Genèvre group is given in Professor Bonney's paper, “Two Traverses of the Crystalline Rocks of the Alps”: Q.J.G.S., 1889, p. 80.Google Scholar The limestone at the western end is marked Jurassic, probably on the strength of Lory's map as Lias compacte or calcaire Briançonnais. It is now included in the Trias.
page 15 note 1 The small-grained tabular gneiss (gneiss minuto tabulare) is extensively quarried in the Susa, Chisone, and Pellice Valleys for building purposes. in Turin.
page 15 note 2 Vasseur and Carez assign the crystalline schists west of Monte Viso on the Italian side to the Palæozoic (Cambrian), for which, however, there is no warrant, the absence of the Lower Palæozoic in the Western Alps being, on the contrary, an important feature as marking a long interval of erosion which led up to the Carboniferous formation, composed of the sedimentary and calcareous products of that erosion in which were engulfed the vast débris-accumulations of a luxuriant vegetation. The Archæan age of the crystalline schists was, after Zaccagna's publication in 1885, affirmed also by Professor Bonney (1886 and 1889), in relation to the Alps generally.
page 16 note 1 In Piémont alone the crystalline schists, lying between the Mercantour massif in the south and Monte Rosa in the north, cover an area of 200 by 30 kilometres, or roughly 2,400 square miles, of which the three principal pietra verde masses represent about one-fifth. These masses are all composed of basic rocks, more especially of diorite, diabase, gabbro, serpentinous and hornblendic rocks. The white marble of Susa is, as shown above, Archæan, in contrast to the Triassic marble of the Apuan Alps, but both attest the process of the deposition of coarse calcareous material being followed by that of gradually finer to very fine material purified by solution and precipitation. The majestic triumphal arch at Susa shows that the marble of that locality, as that of Carrara, was quarried already by the Romans. Similar saccharoidal limestone intercalations are also worked in the Pellice, Upper Po, and Varaita Valleys. I propose to refer to these and the pietra verde areas, as also to Franchi's recent divergent views as to their age, in a subsequent paper.
page 16 note 2 Diener, C. outlines a similar series of belts in his Gebirgsbau der Westalpen, 1891, but embraces in his generalizations the entire chain of the Alps.Google Scholar
page 17 note 1 Of this post-Liassic uprise, followed by a period of erosion, evidence is afforded by a general and marked discordance between the strata of the Upper Lias and the Tithonian, and, again, between the Neocomian and Senonian both in the Western and in the Apuan Alps.