M. C. Deville on the origin of Granite. 181 



of eruptivity in the manner of sulphur, at the limit of which we 

 find the mineral waters and their sources. But these experi- 

 ments do not directly apply, in my opinion, to the case in which 

 quartz, like that of the granites, is a substance eruptive in the 

 manner of the lavas. For the silica obtained by these philoso- 

 phers has always been crystallized ; and it is superfluous to show 

 that it would be false to assume that crystallized silica could not 

 be obtained in any other way. 



Supported by the possible and even probable superfusion of 

 quartz, M. Fournet has endeavoured to explain how that this 

 body in a great many cases, more especially in granite, solidified 

 after other substances which were more fusible than itself. From 

 well-known facts, M. Elie de Beaumont has shown* that there 

 was nothing improbable in such a hypothesis. If Mr. Faraday, 

 working with small quantities and under unfavourable conditions, 

 could retard the solidification of sulphur by nearly 100 degrees 

 (its fusing-point being 109°), what is there astonishing in the 

 assumption that quartz, which perhaps melts at 2000 to 2500 

 degrees, could be maintained in the soft state as low as 1000 to 

 1200 degrees, and that under conditions which must be assumed 

 to be favourable ? I will add that M. Fournet' s opinion, which, 

 when he published it, was only an ingenious hypothesis, seems 

 to me to have become much more probable since it has been 

 found that pure quartz is eminently well adapted to form a glass, 

 that is to say a superfused substance, and that up to the present 

 time it is the substance which presents the greatest difference 

 between its density in the crystalline, and its density in the 

 amorphous state. I therefore cannot see in that an absolute 

 objection to the eruptive origin of granite. 



" But," the able author whom I contend with would remark, 

 ' ' how can it be conceived that minerals which, like felspar, only 

 contain 60 to 65 per cent, of silica, or like mica, which is still 

 more, basic, could be separated from a fused mass in which the 

 granite is in excess ? Evidently this body, which at this tempe- 

 rature acts powerfully on the bases which constitute these mine- 

 rals, would not have allowed those substances to crystallize out 

 without combining with them." 



To refute this objection, it is simply necessary to assume with 

 M. Delafosse, that, in reference to minerals of igneous formation, 

 silica plays a part analogous to that which water plays in refer- 

 ence to substances formed in it. In the latter case definite hy- 

 drates are formed, frequently poor in water. There is thus a 

 class of phenomena in which affinities find their limits. Further, 

 M. Senarmont's ingenious experiments prove that elevation of 

 temperature always produces a tendency to dehydration, even in 

 * Loc, cit. p. 1305, 



