ON THE VAKIOLITIC KOCKS OF MONT GENEVKE. 329 



specimen of typical variolite, brought by the late Prof. Carvill 

 Lewis from Masasco, in Liguria ; and we have recently received 

 from Mr. E. E. Parkinson, A.R.S.M., an excellent example from 

 Monte Penna, on the western frontier of Emilia. 



Though this does not exhaust the list of recorded localities of 

 variolite as pebbles, there are but very few cases known of its 

 occurrence in situ, viz. : the Pichtelgebirge *, Schonfels in Yoigt- 

 land t, and the Harz J in Germany ; Yalguba, near Olonetz, in 

 Russia; Mont Genevre, Cesana Torinese §, Monte Loreto ||, the 

 Rochers de Rioubreut ^, south-west of Monte Viso, and a few 

 other Italian localities. 



Most authors agree that it is a structural modification of a basic 

 rock, whether euphotide, as was maintained by many French 

 geologists, as Pournet, Lory, Hebert, Gras, Cordier, d'Orbignjs and 

 Levy, or of aphanite or diabase, as was held by Haiiy, Lessen, 

 Giimbel, Loewinson-Lessing, Rosenbusch, and others. 



As to the nature of the modification, opinions difi'er materially. 

 The most unsatisfactory modern theory is that which regards the 

 varioles as included and metamorphosed fragments — a view advanced 

 by Giimbel **, and still held by him ft in spite of the criticisms of 

 Zirkel tt and others. Chierici §§ maintained that the varioles were 

 formed by decomposition of garnets, which at first broke down into 

 black granules, and these, in a later stage, were bleached into the 

 typical varioles. The opinion, however, that is most generally re- 

 ceived regards variolite as the product of contact-alteration — a view 

 that has been accepted by, amongst others. Lor}*, Levy and Pouque, 

 Dathe, Leppla, Zirkel, Teall|||i, and Rosenbusch ^% The last- 

 named author concludes that the opinion that variolite arises as an 

 " endomorphe Contactform der Diabase" hardly requires repe- 

 tition. There can be no doubt that many variolites have been 

 formed in this manner; but some, no doubt, have arisen as a 

 crust on lava. Loewinson-Lessing clearly recognizes *** that both 

 methods of formation have been in operation, as he divides the 

 variolites into two classes. He includes among the first group, or 

 contact-modification products, those of the Fichtelgebirge, Pranken- 



* C. W. Giimbel, ' Die palaolitliischen Eruptivgesteine des Pichtelgebirges/ 

 Miinchen, 1874, p. 31 ; N. Jahrb. 1876, ]:)p. 42, 43 ; Geognostische Bescbreibung 

 des Konigreichs Bayern, Abth. iii. (Gotlia, 1879), pp. 213-8. 



t P. Zirkel, Ber. d. k.-sachs. Ges. d. Wiss. xxvii. (1875) p. 211. 



+ K. A. Lossen, Jalii-b. d. k. preuss. geol. Landesanst. fiir 1880, pp. 10-12. 



§ B. Gastaldi, Descriz. della Carta geol. Italia, ii. pt. 2 (Pirenze, 1874), p. 28. 



II Mazzuoli and Issel, Boll. Comit. geol. Italia, vol. xii. (1881) p. 329. 



•y D. Zaccagna, Boll. E. Comit. geol. Italia, xviii. (1887) p. 387. 



** C. W. Giimbel, N. Jahrb. 1876, p. 42. 



tt C. W. Giimbel, Geologie von Bayern, Th. i. Lief. 1. 1884, pp. 78, 79. 



tl P. Zirkel. Ber. d. k.-sachs. Ges. d. Wiss. xxvii. (1875) p. 211. 



§§ Chierici, Ann. des Mines, v. (1820) p. 398. 



II 11 J. J. H. Teall, ' British Petrography ' (London, 1886) p. 134. 



•fl^ H. Eosenbusch, ' Mikroskopische Physiographie dermassigen Gesteine,' 

 Aufl. 2, Bd. ii. (1887)p. 227. 



**^ F. Loewinson-Lessing, " Olonetzkaya Diabazovaya Formatziya," Trud. 

 St. Peterburgskagho Obshcb. Estest. xviii. (1888) p. 169. 



