On the Variolitic Rocks of Mont Genf.Vre. 295
Total Page：16
File Type：pdf, Size：1020Kb
Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on February 23, 2016 ON THE VARIOLITICROCKS OF MONT GENF.VRE. 295 18. The VARIOLrrlc ROCKS of MONT GESkVRE. By GRE~'~VlLLE A. J. COLE, Esq., F.G.S., and ,1. WA.LTEa GREGORY, Esq., F.G.S., F.Z.S. (Read February 5, 1890.) [PLATE XIII.] CONTENTS. I. Introduction. II. Previous work on the Variolite of the Durance. III. General features of the surface. IV. Rocks found in the Oondrau Valley. V. The Gabbro and associated Serpentines. VI. The Dykes. VII. The Variolitie Diabases and Tufts. VIII. Relations of the rocks to one another and to the stratigraphical series. IX. Conditions of :Formation. X. Conclusions of previous authoes as to the nature of the Variolites of other localities. XI. Summary of conclusions. 1. INTRODUC$IOI% THOWH the series of structures found in the acid igneous rocks has now an almost complete parallel in the basic group, a few gaps in the latter still remain unfilled. The spherulitic and perlitic structures of ordinary rhyolites are now well known in the basic lavas ; but the only rock that seems homologous with the pyromerides of Jersey and North Wales is that known as the Variolite of the Durance, which has been described as the selvage of a compact euphotide. Impressed by the perlitic and other structures of this rock, one of us suggested in 1888 * that it was a devitrified tachy- lyre; while a few weeks later, the other attempted to settle the question of its true nature by an examination of the rock in the field. As a series of heavy snowstorms prevented anything being done on that occasion, we visited Mont Gen4vre together last August, to examine the igneous mass to the south of the pass, which had been mapped by Lory as an intrusive euphotide, and from which it was probable that the variolites of both the Durance and the Dora had been derived. "[I. PREVIOUS WORK ON THE VARIOLITE OF THE DURANCE. Variolite is a rock that has been of interest since the time of the Romans on account of its supposed medicinal value; its European sources, however, were lost till it was found near Lucca, early in the 17th century; previously the supply had been drawn from America, where it had long been known as ,' gamaicu." Specimens of the "lapis variolatus" of Lucca were * G. A. J. Cole, " On some additional Occurrences of Taehylyte," Quart. Journ. Geol. See. vol. xliv. p. 307. Q. J. G. S. No. 182. x Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on February 23, 2016 296 MESSRS. G. A. J. COLE A.W'D J. W. GREGORY figured and described by Aldrovandus * in 1648, but no definite opinion as to the nature of the rock of this locality can be obtained from his work. A figure, however, given by Langius t, in 1705, of a "variolites Lucernensis" from the Emmenthal is unmistakable. The rock was subsequently described from other localities, as by Bruckmann + in 1734 and by Demeste w from the bed of the Durance in 1779. Wallerius J] in 1747, and Bertrand 82in 1763, published classifications of Variolite, as by this time different rocks had been included under the same name. Hitherto pebbles only had been found, but in 1775 the kbb6 Rozier's ' Journal' ** announced its discovery in situ in the " ServiSres " valley, by M M. Faujas de St. Fond and Guettard ; the former, however, gave in 1781 "~t an account of his observations, and from this it is obvious that he and his companion had only found fallen masses of rock on the talus-slopes; in the same contribution Faujaa called the Durance rock " lapis variolatu.~ viridis verus," to distinguish it from the amygdaloids that had been confounded with it. Valmont de Bomare's discovery of native silver in the Variolite of the same district led him to examine it, with the result that he contended for ++++ the common origin of varioles and ground-mass (or "variolin " as it was called by Delamdtherie). Though Buffon w167had in 1786 clearly indicated Mont GenGvre as the source of variolite, Morozzo !1[[ threw doubt on its occurrence there. Fournet's elaborate " Mdmoire sur la gdologie de la partie des klpes comprise entre le Valais et l'Oisans," 82182contained an important addition to the knowledge of variolit~ : from his de- scriptions 9** he seems to have collected, probably from the bed of the Durance, compact diabase, perlitic variolite, and porphyritic diabase. Erie de Beaumont probably found variolite in sittt, but no record of his observations seems to have been published, and 9Aldrovandus, UI.,' Musmum MetaUieum in libros iiii. distributum ;' Bononi~ : 1648 : p. 883. r Langius, Carl 5Tie., 'l~istoria Lapidum figuratorum Helvetim ejusque vieinim ;' Lucerna~ : 1705, pp. 40-1. Bruckmann, ' Centuria Epistolarum Itinerarium,' vol. i. ep. xxxi. Wolf- ienbuttel~e, 1742. w ' Lettres du Doct. Demeste au D. Bernard sur la Chymie, Doeimasie,' &e. Paris, 1779 (quoted by Buffon). t[ Wallerius, 'Mineralogia, Eller Mineral-Riket, Indelt och beskrifvit,' Stockholm: 1747, p. 409 (French edition, 1753). 82E. Bertrand, ' Dietionnaire raisonn6 universel des foaailes propres et des fossilea accidentels,' Le lZlage. 1763, t. ii. pp. 238-9. 9* "Observations sur la physique," t. xvi. (1775) p. 517. r Faujas de St. Fond, ' l~istoire Naturelle de la province de DauphinG,' Grenoble : 1781, t. i. ~). 253. See also Faujas de St. Fond, ' Histoire Naturelle des Roehes de Trapp, ed. 2 (Paris, 1813), p. 26, footnote. +§§ ' Dictionnaire raisonn6 universel d'Itistoire Naturelle,' t. viii. Lyon : 1791 * w167' Histoire Naturelle des MinGraux; sur la variolite du PiGment,' t. iv. (Paris, 1786) pp. 395-8. II [[ MGm. Ac~ud. Roy. Sciences (Turin), vol. v. 1793, pp. 16o-72. ~82Ann. des Sci. Physiques et Nat. See. l~y. d'Agrie. Lyon, sGr. i. t. iv. (1841) pp. 105-183, 483-560; ix. (184(;)pp. 1-112; sGr. ii. t. i. (1849) pp. 185- 2(i9, pl. i. 9** Fournet, ibid. sdr. i. t. iv. p. 155. Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on February 23, 2016 ON THE VA1RIOLITICROCKS OF MONT GEN~VRE. 297 his collection from Mont Gen~vre was described by Delesse~; Scipion Gras certainly examined it in situ, and in consequence denied r the volcanic origin of either the variolite or any of the rocks asso- ciated with it. Delesse's memoir " Sur la variolite de la Durance "++ was the first serious attempt to treat the rock from a modern point of view ; the paper was illustrated by a eoloured plate, which was supple- meuted by an exquisite lithographic drawing in the same author's "Recherches sur les roches globuleuses" w Delesse was led by his examination of the collection and acceptance of the field-work of M. Elie de Beaumont to accept the passage from euphotide to ser- pentine, and to regard the variolite as a selvage to the euphotide. Supplementing the laboratory-researches of Delesse, Lory made in the field a series of observations which have long appeared to be conclusive. In 1861 the Geolo~cal Society of France, under his guidance, spent one day in the variolitic area; visiting the Col du Gondran, they saw the largest gabbro mass, and having probably met with this rock again in the Chenaillet Valley, they greatly exaggerated its importance II. Lory argued on that occasion, in opposition to :~[. Hdbert, that the melaphyre-like rock was only an "euphotide porphyroi'de, comme la variolite serait une euphotide globulaire," and that the lat~er was a structural modification of the gabbro on its selvages. These views Lory indicated clearly in 1863 in his map and sections of the Brian~onnais 82;in the latter he repre- sented this igneous massif as composed of euphotide intrusive in the surrounding schists, and with a selvage of variolite on the west and of serpentime on the east. In 1864 he repeated these views in a rather detailed account of the area in question ** ; in his well-known memoir he reaffirmed the passage from euphotide to serpentine on the one hand and to porphyry and variolite on the other tt. The first application of the microscope to the study of thin sections of the variolite of the Durance was made by Inostranzev in 1874, in his valuable paper " O Variolitye "++,++ in which he described in detail the variolite of a new locality, Yalguba in Olonetz, and devoted several pages and figures to the microscopic structure of that of * Ann. Mines, s4r. 4, t. xvii. p. 130. "t" Gras, Sc., " Introduction s uu Essai sur la constitution gdologique des Alpes centrales de la France et de la Savoie," Bull. Soc. gdol. France, 2me s4r. t. i. pp. 723-4. ++ Ann. des Mines, s~r. 4, t. xvii. pp. 116-131, pl. i. Compt. Rend. xxx. pp. 741-743 ; Bull. Soc. g4ol. Ft. 2 "e sgr. t. vii. pp. 427431. .~ M~m. Soc. g~ol. Franc, 2 m" s4r. t. iv. (1852) pl. xxiv. ft. 18-20. J] "R4union extraordinaire "s St. Jean de Maurienne," Bull. Soc. g6ol. France, '2"" s4r. t. xviii, pp. 779-784. 82 "Carte et coupes g~ologiques du Briangonnais," Bull. Soc. g6ol. France, 2 ~~ s4r. t. xx. pp. 233-235, pls. iii. & iv. ** Lory, "Description g4ologique du Dauphin4," Bull. Soc. Statistique de l'Is~re, s~r. 2, t. v. (1861) pp. 1-~ pl. i. ; s4r. 2, t. vi. (1861) pp. 1-26t:, pls. ii., iii. ; s~r. 2, t. viii. (1864) pp. 7-252. +t Loc. cit. par. 293 ; Bull. Soc. Stat. Is~re, s~r.