ON THE VARIOLITIC KOCKS OP MONT GENEVRE. 331 



reason for regarding the spherulitic tachylyte of Carrock Fell * as a 

 variolite, since the basis is not devitrified. The minute size of the 

 spherulites would also debar Sordawalitef from ranking among the 

 true variolites t- 



XI. Summary of the Conclusions arrived at in this Paper. 



We believe, finally, that the discussion of the observations of 

 others and of ourselves justifies the following conclusions : — 



The gabbro or euphotide south of Mt. Genevre is associated with 

 serpentines, which were originally peridotites, and were not derived 

 from the alteration of the gabbro. These coarsely crystalline rocks 

 probably form a considerable subterranean mass, but have little 

 importance at the surface. 



They were broken through by dj^kes of dolerite and augite- 

 andesite, and are now overlain by a great series of compact diabases 

 and fragmental rocks, which has no direct connexion with the 

 gabbro. 



The variolite of the Durance occurs in situ as a selvage on the 

 surfaces of these diabases among themselves ; as blocks in the 

 fragmental rocks, which are regarded by us as tuffs ; and occa- 

 sionally as a selvage to the diabase dykes. 



This product of rapid cooling was originally a spherulitic tachy- 

 lyte, and has become devitrified by slow secondary action. Variolite 

 stands in the same relation to the basic lavas as pyromeride does to 

 those of acid character. 



The eruptive rocks in the Mt.-Geuevre area are probably post-Car- 

 boniferous; but their exact age cannot at present be determined. 



There are several other areas of similar variolitic rocks among 

 both the Alps and the Apennines of Piedmont and Liguria. 



The best modern representative of the conditions that produced 

 these rocks is to be found in the great volcanoes of Hawaii; and there 

 is nothing, either in their fundamental characters or in their mode 

 of origin, that cannot be paralleled among the products of causes 

 now in action. 



In the preparation of this paper we have been guided at many 

 points by the kind advice of Prof. Bonney and Prof. Judd. We 

 have thus also been able to avail ourselves of several specimens and 

 sections in the collections of the Normal School of Science and Royal 

 School of Mines. Our indebtedness to those who have preceded us 

 on the classic ground of Mont Genevre is, we trust, fully apparent 

 in the paper. 



* T. T. Groom, Quart. Journ. Geol. Soc. vol. xlv. (1889) p. 303. 



t F. Loewinson-Lessiiig, " Die mikroskopische Beschaffenbeit des Sordawa- 

 its," Tsch. Min. u. petr. Mitth. ix. (1887) pp. (U-76. 



I [By the kindness of Prof. Blake we have been enabled to examine the vario- 

 lite found by bim near Careg Gwladjs in Anglesey (Brit. Assoc. Kep. 1888, p. 11, 

 pi. Y. fig. 22) ; as this is the first discovery of the rock in the British Isles, we 

 hope it will be more fully described.] 



