Igneous Mocks and Volcanos. 197 



tive among the ejected products of the central fire, seems a hypo- 

 thesis not only unnecessary, but when we consider their varying 

 composition, untenable. 



We are next led to consider the nature of the agencies which 

 have produced this plastic condition in various crystalline rocks. 

 Certain facts, such as the presence of graphite in contact with 

 carbonate of lime, and oxyd of iron, not less than the presence of 

 alkaliferous silicates, like the feldspars in crystalline limestones, foi- 

 bid us to admit the ordinary notion of the intervention of an intense 

 heat such as would produce an igneous fusion, and lead us to 

 consider the view first put forward by Poulett Scrope, * and since 

 ably advocated by Scheerer and by Elie de Beaumont, of the 

 intervention of water aided by fire, which they suppose may com- 

 municate a plasticity to rocks at a temperature far below that 

 required for their igneous fusion. The presence of water in the 

 lavas of modern volcanos led Mr. Scrope to speculate upon the 

 effect which a small portion of this element might exert at an 

 elevated temperature and under pressure, in giving a liquidity to 

 masses of rock, and he extended this idea from proper volcanic 

 rocks to granites. 



Scheerer in his inquiry into the origin of granite has appealed 

 to the evidence afforded us by the structure of this rock, that the 

 the more fusible feldspars and mica crystallized before the almost 

 infusible quartz. He also points to the existence in granite of 

 what he has called pyrognomic minerals, such as allanite and 

 gadolinite, which, when heated to low redness, undergo a peculiar 

 and permanent molecular change, accompanied by an augmenta- 

 tion in density, and a change in chemical properties, a phenomenon 

 completetly analogous to that offored by titanic acid and chromic 

 oxyd in their change by ignition from a soluble to an insoluble 

 condition. These facts seem to exclude the idea of igneous fusion, 

 and point to some other cause of liquidity. The presence of 

 natrolite as an integral part of the zircon-syenites of iNorway, 

 and of talc and chlorite and other hydrous minerals in many 

 granites show that water was not excluded from the original granitic 

 paste. 



Scheerer appeals to the influence of small portions of carbon 

 and sulphur in greatly reducing the fusing point of iron. He 

 alludes to the experiments of Schafhautl and Wholer, which show 



* See Journal of Geol. Society of London, vol. xii. p. 326. 



