6 THEORY OF IGNEOUS ROCKS AND VOLCANOES. [I. 



He recalls the aqueous fusion of many hydrated 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 dis- 

 cussion with Durocher,* is strongly confirmed hy the late ex- 

 periments 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 wollastonite, 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 oxide of iron, the wollastonite was replaced by crys- 

 tals of diopside. Obsidian in the same manner yielded crystals 

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

 these experiments upon vitreous alkaliferous matters, the pro- 

 cess of nature in the metamorphosis of sediments is reversed ; 

 but Daubree foijnd 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 communicated 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 min- 

 erals 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 fu.si"ii. 



I had already shown in the Report of the Geological Sur- 



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

 France, Second Series, Vol. IV. pp. 468, 1018; VI. 644; VII. 276; VI I r. 

 600; also, Elie de Beaumont, Ibid., Vol. IV. p. 1312. See also the re- 

 cent microscopical observations of Mr. Sorby, confirming the theory of the 

 aqueo-igneous origin of granite in the L. E. & D. Phil. Mag., February, 

 1858. 



