182 M. C. Deville on the origin of Granite, 



a liquid medium. For instance, gelatinous silica entirely loses 

 its water, and becomes deposited as crystalline quartz ; and a 

 solution of sesquichloride of iron decomposes into hydrochloric 

 acid and anhydrous sesquioxide. 



My brother*, in his researches on the phenomena of disso- 

 ciation, has cited still more curious facts : oxygen and potassium 

 not only do not combine, but at a high temperature cannot 

 remain combined. In the presence of facts like these, it is not 

 astonishing that silica in a fused state should act as mother- 

 liquor to the felspar, only combining with the other elements 

 within limits appropriate to the physical and chemical pro- 

 perties of the granitic bath. 



One of the objections which M. Rose raises against the 

 eruptive origin of granite, depends on the remarkable properties 

 of the so-called pyrognomic minerals. M. Scheerer, who has 

 profoundly studied this class of phenomena, has perfectly well 

 seen that to found on them an absolute argument against the 

 eruptive origin of granite is to exaggerate their importance. He 

 has even been led to conclude that the solidification of the rock 

 has taken place under special physical conditions. Indeed M. 

 Rose himself, who first presented this objection, does not seem 

 to attribute to it any considerable value; for he addsf, u It may 

 be assumed that these minerals, more especially gadolinite, are 

 produced by fusion while associated with granite, but that by 

 the prolonged action of the atmosphere, of wi.ter, of an ele- 

 vated temperature, and other influences they have changed this 

 condition." 



If, as M. Rose observes, such a hypothesis is not opposed 

 to the ideas which he endeavours to enforce in his memoir, still 

 less is it opposed to those which I advocate. 



But if we in turn examine the defence of the partisans of the 

 Neptunian origin of granite J, how are we to explain the division 

 of the different acid and basic elements, between the micas 

 and the felspars ? Why should there be formed at one 'time 

 two micas ? and more especially two felspathic minerals having 

 two formulae essentially different, orthose and oligoclase? 

 Evidently nothing that takes place in our laboratory solutions 

 can give the key to these natural phenomena. Let us admit, 

 then, that between the different elements of the rocks there 



* Comptes Rendus, vol. xlv. p. 489 ; Phil. Mag. vol. xvi. p. 516. 



t Phil. Mag. vol. xix. p. 39. 



X I use the expression Neptunian origin employed by M. Rose, and not 

 that of aqueous origin. The latter, m fact, cannot be the antithesis of 

 plutonic or eruptive origin. I shall show further, that not only is water 

 present in eruptive phenomena, but there is actually no eruption in which 

 it does not play an important chemical part. 



