ON THE THEORY OF IGNEOUS ROCKS AND VOLCANOS. 205 



sulphur in greatly reducing tlie fusing point of iron. He alludes to 

 the experiments of Schafhautl and "Wohler, which show that quartz 

 and apophylite may be dissoh'ed by heated water under pressure and 

 recrystallized on cooling. He recalls the aqueous fusion of many hy- 

 drated salts, and finally suggests that the presence of a small amount 

 of water, perhaps five or ten per cent., may suffice at a temperature 

 which may approach that of redness, to give to a granitic mass a 

 liquidity, partaking at once of the characters of an igneous and 

 an aqueous fusion. 



This ingenious hypothesis, sustained by Scheerer in his discussion 

 with Durocher,* is strongly confirmed by the late experiments of 

 Daubree. He found that common glass, a silicate of lime and alkali, 

 when exposed to a temperature of 400° C, in presence of its own 

 volume of water, swelled up and was transformed into an aggregate of 

 crystals of woUastonite, the alkali with the excess of silica separating, 

 and a great part of the latter crystallizing in the form of quartz. 

 When the glass contained oxyd of iron, the woUastonite was replaced 

 by crystals of diopside. Obsidian in the same manner yielded crys- 

 tals of feldspar, and was converted into a mass like trachyte. In these 

 experiments upon vitreous alkaliferous matters, the process of nature 

 in the metamorphosis of sediments is reversed, but Daubree found 

 still farther that kaolin, when exposed to a heat of 400'^ C. in the 

 presence of a soluble alkaline silicate, is converted into crystalline 

 feldspar, while the excess of silica separates in the form of quartz. 

 He found natural feldspar and diopside to be extremely stable in the 

 presence of alkaline solutions. These beautiful results were communi- 

 cated to the French Academy of Sciences on the 16th of November 

 last, and as the author well remarked, enable us to understand the 

 part which water may play in giving origin to crystalline minerals in 

 lavas and intrusive rocks. The swelling-up of the glass also shows 

 that water gives a mobility to the particles of the glass at a temperature 

 far below that of its igneous fusion. 



I had already shown in the Report of the Geological Survey of 

 Canada for 1856, p. 479, that the reaction between alkaline silicates 

 and the carbonates of lime, magnesia and iron at a temperature of 

 100° C. gives rise to silicates of these bases, and enables us to explain 

 their production from a mixture of carbonates and quartz, in the 

 presence of a solution of alkaline carbonate. I there also suggested 



* Note.— See for the arguments on the two sides. Bulletin of the Geol. Soc. of France, 

 Second series, vol. iv., p. p. 468, 1018 ; vi., 644 ; vii., 276 ; viii., 500 ; also, Elie de Beaumont, Ibid, 

 vol. iv., p. 1312. See also the recent microscopical observations of Mr. Sorby, confirming the 

 theory of the aqueous-igneous origin of granite.— i. E- & D. Phil. Mag., February, 1858. 



