316 MESSRS. G. A. J. COLE AND J. W. GEEGORY 



If, then, the variolite represents a vitreous selvage-producfc, how 

 comes it to be so widely diffused among the masses of the diabase- 

 lavas? It is possible that the surfaces of ordinary splieroids of 

 contraction, even in the heart of a cooling mass, may differ appre- 

 ciably from the more central portions, and, consolidating more 

 rapidly, exhibit a vitreous structure. Delesse * thus concluded, 

 from a number of determinations of specific gravity, that the centres 

 of prisms and spheroids in igneous rocks are denser than the ex- 

 ternal laj^ers. If denser, even a glassy crust may occur outside 

 while the core is fairly crystalline. But we prefer to read in the 

 irregular shape and involuted surfaces of the diabase-masses £>f Mt. 

 Genevre evidence of the rolling over of lavas among themselves ; 

 and we are led to regard the presence of variolitic selvages through- 

 out such great thicknesses of rock as largely due to movements 

 taking place within a crater, a point to which Ave shall revert in 

 Section IX. That the mode of occurrence of the variolite of Mt. 

 Genevre is paralleled at other places may be seen by the descriptions 

 of Mazzuoli and Issel t of kidney-shaped masses of aphanite near 

 Bonassola in Liguria, which have variolitic surfaces of junction. 

 ZaccagnaJ also records similar phenomena in a spheroidal diabase 

 near Mte. Viso. 



A glance at the sketch-map of the district (p. 300) will at once show 

 the large area covered by the variolite-diabases. When we add that 

 the thickness of these compact rocks often amounts to 300 metres, 

 and appears to reach 500 metres in the neighbourhood of Mt. La 

 Plane, some idea may be formed of the importance of the series. In 

 places both folds and faults are traceable ; but we believe that the 

 thicknesses here stated are not the result of actual repetition. 



Our map also indicates the occurrence, particularly upon Le Che- 

 naillet, of rocks styled by us Yariolite-Tuffs. These, as has been 

 stated, readily attract attention among the pebbles in the bed of 

 the Durance, though, with an exception in the Museum of Grenoble, 

 we have not found specimens of them in any public collection. 

 Delesse §, however, cites Elie de Beaumont as stating (probably in 

 conversation) that " I'euphotide formant le massif central du Mout- 

 Genevre est encore accompagnee ou entouree par des roclies brechi- 

 formes varie'es, ainsi que par des roches probablement metamor- 

 phiques." Cordier || has, moreover, in his classification the lieading 

 " Breche variolitique," and he describes one of his specimens as 

 follows : — " Breclie de variolite embryonnaire. Cettc rochc, tres- 

 curieuse, ayant la memo origine que toutes celles que j'appcUe 

 BrecJie de froissement, sert d'epontea la variolite qui forme des amas 

 transversaux dans les serpentines des parties superieures de la 

 vallee de Servieres (Hautes Alpes)." It is possible that the pow- 



* ' Metainorpliisme des Eocbes,' Paris, 1858, pp. 371-4. 

 t Boll. E. Comit. geol. d'ltalia, vol. xii. (1881), p. 329. 

 t Ibid. vol. xviii. (1887), p. 387. 



I Ann. des Mines, 4*= ser. t. xvii. (1850), p. 130. 



II ' Description des Eocbes,' 1868, p. 173. 



