M. C. Deville on the origin of Granite. 179 



while silicic acid of spec. grav. 2*65 would in all cases be the 

 product of slow and gradual actions. 



The concurrence of the two facts may be readily understood ; 

 gelatinous silica, cemented by solidified silica, would have a den- 

 sity intermediate between the two. 



M. Rose rightly remarks that none of the natural varieties of 

 quartz of the density 22 appear to owe their molecular state to 

 fusion. And as the quartz fused in the laboratory in small 

 pieces is always vitreous, and of this low specific gravity, it must 

 be concluded that the quartz of granite has never been in a 

 state of igneous fusion. With regard to the felspar of granite, 

 a similar conclusion must be drawn frpm the fact that the expe- 

 rimenters who have succeeded in melting this body have never 

 been able to obtain it crystallized, but always in a state of glass. 



All geologists will understand the hesitation I feel at follow- 

 ing this able chemist in his discussion on the origin of granite. 

 Not that the question appears to me as it does to him, as a barren 

 one. On the contrary, I am convinced that it is as fruitful and 

 as well deserving attention as any of those which are connected 

 with the habitable globe. 



I cannot, however, but remark that the foregoing conclusions 

 cannot be legitimately deduced from the negative facts observed 

 in the laboratory. For my own part, I confess that if any one 

 had thus argued when I passed over the lava of Arso, which 

 burst out in 1308 in the Island of Ischia, and which is full of 

 the well-known beautiful felspars, it would have been difficult to 

 convince me that nature has not been able to form felspar* by 

 igneous action. 



I will say as much for the micas of Vesuvius, and of those of 

 the lava and scoria of the Laacher See ; and, supposing even that 

 they did not contain a trace of fluorine, an element so variable in 

 the micas of granite, nor the small quantity of water the exist- 

 ence of which is very problematical in most cases, I should boldly 

 conclude that if the mica has in the above cases an eruptive 

 origin (and for a geologist there cannot be the slightest doubt), 

 the micas of granite have also been formed in the same manner. 



This kind of negative argument cannot therefore be used to 

 combat the eruptive origin of two of the elements of granite. 

 There remains quartz : M. Rose (after many able chemists, and 

 particularly MM. Fuchs and BischofF) offers several objections to 

 its eruptive origin. 



The first depends upon the negative results of the laboratory 

 — that a glass, and not a crystalline quartz, is always obtained by 



* By felspar I understand orthose, which is the prevalent felspar in gra- 

 nite. What I say is much more true for oligoclase, and more especially for 

 Labradorite, which is so abundantly found in modern lavas. 



