I.] THEORY OF IGNEOUS ROCKS AND VOLCANOES. 5 



in contact with carbonate of lime and oxide of iron, not less 

 than the presence of alkaliferous silicates like the feldspars in 

 crystalline limestones, forbid 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 heat, which they suppose may communicate a plasticity 

 to rocks at a temperature far below that required for their 

 igneous fusion. The presence of water in the lavas of modern 

 volcanoes led Mr. Scrope to speculate upon the effect which a 

 small portion of this element might exert, at an elevated tem- 

 perature and under pressure, in giving 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 ap- 

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

 that 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, accom- 

 panied by an augmentation in density and a change in chemical 

 properties ; a phenomenon completely analogous to that offered 

 by titanic acid and chromic oxide 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 Norway, and of talc, chlorite, and 

 other hydrous minerals in many granites shows that water 

 was not excluded from the original granitic paste. Scheerer 

 appeals, by way of illustration, to the influence of small portions 

 of carbon and sulphur in greatly reducing the fusing point of 

 iron. He alludes to the experiments of Schaf hautl and Wohler, 

 which show that quartz and apophyllite may be dissolved by 

 heated water, under pressure, and recrystallized on cooling. 



* See Journal of Geological Society of London, Vol. XII. p. 326. 



