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III.—The “Pietre Verdi” of the Piémontese Alps

Published online by Cambridge University Press:  01 May 2009

C. S. Du Riche Preller
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Extract

In a previous paper on the Permian Formation in the Alps of Piémont, Dauphiné, and Savoy, I referred incidentally to the large masses of pietre verdi or greenstones which constitute perhaps the most striking geological feature of the extensive areas covered by the crystalline rocks of the Piémontese Alps in a crescent-shaped curve about 200 miles in length from the Maritime range to Monte Viso, Grand Paradiso, and Monte Rosa. In the present paper I propose to deal more fully, although necessarily within narrow limits, with these pietre verdi which, owing alike to their extraordinary development, variety, and complexity, to their intimate association with each other and with the crystalline sedimentary rocks, and to their intricate composition and origin, have for the last fifty years presented most interesting problems and passed through many remarkable phases of interpretation. As a necessary preliminary to a description of the different areas, it will be convenient to briefly consider the most recent classification of the crystalline formations of the Piémontese Alps generally, and of the pietre verdi rocks in particular.

Type
Original Articles
Information
Geological Magazine , Volume 3 , Issue 4 , April 1916 , pp. 156 - 163
Copyright
Copyright © Cambridge University Press 1916

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References

page 156 note 2 Geol. Mag., January, 1916, p. 7; ibid., p. 15.

page 156 note 3 Zaccagna, D., “Studi geol. sulle Alpi Occid.”: Boll. R. Com. geol. d'It., 1887, p. 346 et seq.Google Scholar “Riassunto di Osserv. sul Versante Occid. Alpi Graje”: ibid., 1892, p. 175 et seq.

page 156 note 4 Gastaldi, B., “Studi geol. sulle Alpi Occid.”: Mem. R. Com. geol. d'It., 1871, vol. i, p. 3 et seq.Google Scholar“Spaccato geol. lungo le valli sup. Po e Varaita”: Boll. R. Com. geol., 1876, p. 104 et seq.Google Scholar

page 157 note 1 Boll. R. Com. geol., 1887, pp. 342–5.

page 157 note 2 Termier, P., “Sur le Permien du massif de la Vanoise”: Bull. Soc. géol. France, vol. xxi, p. 124 et seq., 1893.Google Scholar

page 157 note 3 Bertrand, M., Bull. Soc. géol. France, vol. xxii, p. 69 et seq., 1894.Google Scholar

page 157 note 4 Parona, C. F., “Sugli Scisti silicei a radiolarie di Cesana presso il Monginevra”: Atti R. Acc. Sc. Torino, vol. xxvii, 17. Gennajo, 1892;Google Scholar also noticed in Davies & Gregory's paper on “The Geology of Mont Chaberton”, Q.J.G.S., 1894, p. 303 et seq.

page 157 note 5 S. Franchi, “Sull' età mesozoica della zona delle pietre verdi nelle Alpi Occidentali”: Boll. R. Com. geol., 1898, pp. 173, 325 et seq. “Ancora sull'età mesozoica, etc.”: ibid., p. 125 et seq. Franchi, Novarese, and Stella were in charge of the detailed survey of the Piémontese Alps for the new 1 : 100,000 map in conjunction with Mattirolo, who supported Zaccagna's interpretation. Franchi published in Boll. R. Com. geol., 1909, p. 252, a forty-page reference of the literature on the crystalline schists from Gastaldi (1871) downwards.

The principal localities which yielded Triassic and Liassic fossils in the calcareous and dolomitic masses of the calc-schist horizon are the Grana, Narbone, Maira, Elva, and Varaita Valleys in Southern Piémont; Chianoc in the lower, and Rocca d'Ambin, Gad d'Oulx, and Bardonecchia in the upper Susa Valley; Villeneuve in the upper Aosta Valley, and the Col du Petit St. Bernard, all in Northern Piémont. The fossils, most of which were determined by Professor Di Stefano and Professor Canavari, include, among others, Radiolaria, Belemnites, Arietites, Crinoids, Encrinus, Pleurotomaria, Avicula, Corallari, Gyropellæ, Pentacrinus, Phylloceras, etc.

page 158 note 1 Zaccagna, D., “Osservazioni sugli ultimi lavori intorno alle Alpi Occidentali”: Boll. Com. geol., 1901, pp. 4, 129; 1902, p. 149; 1903, p. 297.Google Scholar

page 158 note 2 Zaccagna's interpretation agrees with Professor Bonney's view that nothing is more common in the Alps than Jurassic and Triassic wedges in the crystalline schists. “Mesozoic Rocks and Crystalline Schists in the Lepontine Alps”: Q.J.G.S., 1894, p. 285; also ibid., p. 277. Baretti (Studi Gran Paradiso, etc., 1876–7) also considered the French calc-schists the upper and the Piémontese calc-schists as the lower crystalline formation.

page 158 note 3 T. Taramelli and C. F. Parona, “Relazione sull'età da assegnarsi alla zona delle pietre verdi nella Carta geol. delle Alpi Occidentali”: Boll. R. Com. geol., 1911, pp. x–xxiv. The controversy between Franchi and Zaccagna turned more especially on the great calc-schist area extending from the Gesso Valley in Southern Piémont parallel to the Franco-Italian frontier to the Susa and Aosta Valleys towards Monte Rosa. The smaller, isolated area of Courmayeur, running parallel to Mont Blanc, was recognized as Liassic and Triassic, and was, therefore, not in dispute.

page 159 note 1 The official geological map of France, 1 : 1,000,000, published in 1904, which extends to the Italian side as far as the Po Valley, includes in the Permo-Carboniferous not only the minute and tabular gneiss and mica-schists, but also the primitive gneiss belt, for which there is no warrant. Similarly, Termier (“Les schistes cristallins des Alpes occidentales,” Comptes Rendus du Congrès géol. Vienne, 1913) embraces in his série cristallo-phylienne triassique compréhensive all the younger formations down to the Eocene inclusive. Both cases are ultra-synthetic, and are not accepted by the Italian Survey.

Professor Gregory's view that the gneisses which he terms Waldensian (Q.J.G.S., 1894, p. 232 et seq.) are Pliocene and intrusive runs counter to the accepted interpretation of the coarse-grained gneiss being the primitive, viz. ‘fundamental’, substratum of the Cottian and Grajan Alps. Professor Gregory's conclusions are traversed also by Novarese, “Rilevamento geol. Valle Germanesca (Alpi Cozie), 1894,” Boll. R. Com. geol., 1895, p. 277 et seq., and Franchi, ibid., 1897, p. 13 et seq.

page 160 note 1 G. Strüver, “Cenni sui graniti massicci delle Alpi Piémontesi e sui minerali delle valli di Lanzo”: Mem. descr. Carta geol. d'Italia, 1871, p. 37 et seq. M. Baretti, “Studi geol. sul gruppo del Gran Paradiso”: Mem. Acc. Lincei Torino, vol. i, p. 197 et seq., 1876–7. L. Bucca, “Appunti petrogr. sul gruppo del Gran Paradiso”: Boll. R. Com. geol., 1886, p. 449 et seq.

page 160 note 2 The table is founded on the nomenclature worked out by Novarese and Franchi, Boll. R. Com. geol., 1895, p. 164 et seq. and p. 181 et seq.; but I have arranged it somewhat differently so as to group the rocks with primitive and those with secondary elements separately and more prominently.

page 161 note 1 Strüver, Una Salita alle Torre d'Ovarda, Torino, 1873, and Bucca, loc. cit., 1886, p. 453.

page 161 note 2 The felspar-actinolitic rock noticed by Professor Bonney near Fenestrelle in the Chisone Valley (“Two Traverses of the Crystalline Rocks of the Alps”: Q.J.G.S., 1889, p. 80 et seq.) is an amphibolic prasinite, viz. ovardite, while the schist with glaucophane, the epidiorite, and the dark-green porphyrite mentioned by Professor Gregory in his “Waldensian Gneisses”, loc. cit., come under the category of prasinites (ovardites) and amphibolites.

page 162 note 1 Franchi, S., “Alcune metamorfosi di eufotidi e diabasi Alpi Occid.”: Boll. R. Com. geol., 1895, p. 181 et seq.Google Scholar

page 162 note 2 Hence his well-known dictum: “In the Piémontese Alps plutonism is a myth.”

page 163 note 1 Professor Bonney has described an instructive case of conversion of greenstone into schist on a small scale in the Bernina region, Q.J.G.S., 1894, p. 279 et seq.