ON THE YARIOLITIC ROCKS OF MONT GENKVEE. 



295 



18. The Variolitic Rocks of Mont Genevre. By Grenville A. J. 

 Cole, Esq., F.G.S., and J.Walter 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 Gondran Valley. 



V. The Gabbro and associated Serpentines. 

 VI. The Dykes. 

 VII. The Variolitic Diabases and Tuffs. 



VIII. Relations of the rocks to one another and to the stratigrapbical 

 series. 

 IX. Conditions of Formation. 



X. Conclusions of previous authors as to the nature of the Variolites 

 of other localities. 

 XI. Summary of conclusions. 



I. Introduction. 



Though 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 pyroraerides 

 of Jersey and iSTorth Wales is that known as the Yariolite 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- 

 lyte ; 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 sngwstorms prevented anything being 

 done on that occasion, we visited Mont Genevre 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. 



II. 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 

 l.ucca, 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 Tachylyte," Quart. 

 .Tourn. Geol. Soc. vol. xliv. p. .'507. 



a J. G. S. iS^o. 182. X 



