ON THE VARIOLITIC EOCKS OF MONT GENEVRE. 315 



with ordinary spherulites. Loewinson-Lessing *, on the other hand, 

 regards them in a similar rock as globular forms analogous to por- 

 phyritic felspars, a view calculated to lead the mind away from a 

 number of most valuable comparisons. It is clear that the varioles 

 are now largely composed of crystalline fibres ; but it is doubtful if 

 these are all of the same mineral composition. Delesse f has given 

 the following analysis of globules brought by Scipion Gras from 2 

 kilometres south of the village of Mont Genevre : — 



Silica 56-12 



Alumina 17'40 



Oxide of iron 7*79 



Oxide of chromium 0*51 



Oxide of manganese traces 



Lime 8*74: 



Magnesia 3-41 



Soda 3-72 



Potash 0-24 



Loss on ignition 1-93 



99-86 



The globules from a similar specimen from the " torrent de Cer- 

 vieres " yielded M. Levy + results agreeing closely with the 

 above. 



With hesitation, Delesse classed the globules as triclinic felspar. 

 Their specific gravity is given as 2-923 ; M. Levy records 2*920 ; 

 and we have found a large spherule which has a specific gravitv as 

 high as 2-96. 



M. Michel Levy § concludes that oligoclase-fibres form 60 per 

 cent, of the globules, the remainder being composed of amphibole 

 and pyroxene. It is equally impossible for us to refer our own 

 materials to any one mineral, but rather to a mixture, in which 

 felspar largely predominates. The varioles may have originally 

 included only a small amount of glassy matter, and may even have 

 been among the crystalline '' belonospherites" of Vogelsang; yet 

 we cannot on this account cut them off from kinship with the more 

 familiar types of spherulites. Indeed we know how in modern 

 tachylytes pleochroism and other effects of crystallization are ob- 

 servable in the fibres of bodies that are exactly comparable in mode 

 of origin to the spherulites of pitchstone or obsidian. The whole of 

 a basic rock, even to the vitreous selvage, becomes, under similar 

 conditions, more crystalline than the corresponding acid type. The 

 variolites of Mt. Genevre have undergone, in addition, the very 

 extensive secondary devitrification of which the epidote-veins and 

 microlites of actinolite afford such abundant evidence. 



* Tscherm. Min. u. petr. Mitth. Bd.vi. 1885, p. 2*.)8. 



t Ann. des Mines, 4« ser. t, xvii, (1850), p. IIG, Also Coiuptes Reudus, 

 t. XXX. (1850), p. 741. 



+ Bull. Soc. gcol. France, 3^ ser. t. v. (1877), p. 248. 

 I Ibid. p. 250. 



