ON THE VARIOLITIC ROCKS OF MONT GENEVEE. 



327 



of rocks so highly altered. The analysis made by Delesse * of 

 the variolite of Mt. Geiievre corresponds closelj^ with the average 

 composition of a number of Hawaiian lavas, as already quoted 

 in this Journal from the work of Cohen f. But an average of 

 twelve analyses by Silvestri J differs in many respects from that 

 derived from Cohen's figures, the percentages of silica and magnesia, 

 for instance, being distinctly lower. It seems, moreover, from the 

 experiments of Silvestri, that ease of fusion or a refractory cha- 

 racter cannot be ascribed to the predominance in these lavas of 

 any particular base §. We quote these averages for comparison 

 with the variolite, but fully recognize the extensive alteration 

 of the latter. We have excluded the two most altered examples in 

 dealing with the analyses of Silvestri. 



I. 



"Variolite. 



(Delesse.) 



Silica 52-79 



Alumina 11*76 



Oxides of Iron .... 11-07 

 Oxide of Manganese, trace 



Lime 5-90 



Magnesia 9-01 



Soda 3-07 



Potash 1-16 



Oxide of Chromium . trace 

 Loss on ignition . . . 4-38 



99-14 



II. 



Average 

 composition of 

 Hawaiian lavas. 



(Cohen.) 



51-71 



11 



Average 



composition of 



Hawaiian lavas. 



(Silvestri.) 



48-25 



11-44 



15-65 



12-62 



16-59 



10-75 



8-40 



7-59 



3-70 



3-47 



3-62 



0-67 



1-57 



0-98 



0-41 



It is probable, then, that complete fusion, arising from abundant 

 access of water || or unusually high temperature, rather than any 

 chemical peculiarity, will best explain the wide extension of glassy 

 selvages in our area. But the occurrence of the variolite, with its 

 detailed analogy to the tachylytes of the present day, forms the best 

 proof, if proof were needed, of the igneous origin of the diabase- 

 series of Mt. Gencvre. 



X. The Conclusions of Pbeviotjs Authors as to the Nature of 



THE VaEIOLITES OF OTHER LOCALITIES. 



Though variolite is rare and local, a fairly long list of its occur- 

 rences can be compiled. Of these, however, a considerable proportion 

 may be at once dismissed, as there are several very different rocks 



* Bull. Soc. geol. France, 2^ s6r. t. vii. (18.50) p. 430. 



t Quart. Journ. Geol. Soc. vol. xxxix. (1883) p. 457; Cohen, Neues Jahrb. 

 flir Min. &c. 1880, vol. ii. p. 41. 



J "Lave del Vulcano Kilauea," Boll. E. Comit. geol. d'ltalia, vol. xix. 

 (1888) p. 194. 



§ Ibid. pp. 135, 141, 171, &c. 



i| See Prof. Judd, " The Natural History of Lavas," Geol. Mag. 1888, p. 11. 

 Q.'j.G.S. No. 182. z 



