298 MESSRS. G. A. J. COLE AND J. W. GREGORY 



the Durance. These include an excellent coloured drawing of the 

 Durance variolite and a figure that illustrates the corrosion of the 

 varioles by the ground-mass *. Inostranzev fully recognized that 

 the varioles were closely akin to spherulites ; he considered the 

 two structures to be distinguished by the fact that in the variolitic 

 the central sphere of radial fibres is enclosed by a series of concen- 

 tric la^^ers, which do not share in the formation of the black cross 

 under polarized light. 



Zirke] t, in 1875, made a detailed microscopic study both of the 

 variolites of the Durance and of those described by Giimbel from 

 the Fichtelgebirge : he corrected Delesse's statement that the 

 ground-mass of the rock is a felspathic paste, and declared that 

 microscojnc study rendered untenable the view of the connexion of 

 the gabbro and variolite that had been taught by Elie de Beaumont, 

 Cordier, Scipion Gras, Eoth, and others. In the next year, 

 however :|:, he admitted variolite as a contact-product of diabase, 

 and Senft §, in the same year, described it as a diabase-aphanite. 

 In 1877 appeared Levy's important memoir ||, which embodies the 

 most detailed study yet made of the microscopic characters of 

 variolite. Relying on Lory's map and sections, he described it as 

 a compact euphotide formed by the rapid cooling of the selvage of a 

 great euphotide-dyke, thereby diffeiing from Zirkel, and agreeing 

 with Lory and Delesse. He described the fluidal structure, and 

 made the important discovery that some specimens were perlitic, as 

 is well shown in the figure given subsequently in the ' Mineralogie 

 Micrographique ' ^. 



In 1887, in the second edition of his ' Mikroskopische Physio- 

 graphic,' Rosenbusch ** redescribed variolite and accepted it as a 

 selvage-product of a basic rock, analogous to the spherulitic structures 

 so common in the glasses of the acid series. 



In the same year Zaccagna ft, in describing some spherulitic 

 diabases from the neighbourhood of Monte Yiso, referred to the 

 variolites of Mont Genevre as a thin crust on decomposing diabase 

 spheroids, and we regard this description as the truest that has yet 

 appeared. 



Finally, in 1888, the perlitic structure of the rock was again 

 described and figured Xt-, accompanied by the suggestion that the 

 rock was a devitrified tachylyte. 



* Loc. cit. p. 27. 



t " Die Structur der Variolite," Berichte d. k.-sachs. Ges. Wiss. xxvii (1875) 

 pp. 209-220. 



I F. Zirkel, Neues Jahrb. 187(>, pp. 279-280. 



§ F. Senft, ' Synopsis der Mineralogie und Geognosie,' Abth. ii. 1876, p. 556. 



II A. Michel Levy, " Structure et Composition mineralogique de la variolite 

 de la Durance," Coinptes Rendus, t. Ixxxiv. pp. 2()4-266 ; Meuioire sur la vai'io- 

 lite de la Durance," Bull. Soc. Geol. France, ser. 3, t. v. (1877) pp. 232-266. 



*\\ Fouque & Levy, Mineralogie micrographique, (Paris) 1879, pi. xxiv. fig. 2. 



** H. Rosenbusch, 'Mikroskopische Physiographic der massigen Gesteine,' 

 ed. 2, vol. ii. 1887, pp. 227 et acq. 



't D. Zaccagna, " Sulla geologia delle Alpi occidentali," Boll. R. Comit.geol. 

 Italia, xviii. (1887) pp. 387-S. 



+t G. A. J. Cole, Quart. Journ. Geol. Sec. vol, xliv. p. 366-7, pi. xi. fig. 6. 



