204 ON THE THEORY OF IGNEOUS ROCKS AND VOLCANOS. 



trate like a veritable trap into fissures in the quartzite and gneiss. A 

 rock of sedimentary origin may then assume the conditions of a so- 

 called igneous rock, and who shall say that any of the intrusive granites, 

 dolerites, euphotides, and serpentines, have an origin distinct from the 

 metamorphic strata of the same kind, which make up such vast por- 

 tions of the older stratified formation ? To suppose that each of these 

 sedimentary rocks has also its representative among the ejected pro- 

 ducts of the central fire, seems a hypothesis not only unnecessary, hut 

 when we consider their varying composition, untenable. 



"We are nest 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 sili- 

 cates, 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 fire, 

 which they suppose may communicate a plasticity to rocks at a tem- 

 perature 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 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 more 

 fusible feldspars and mica crystallized before the almost infusible quartz. 

 He also points to the existence in granite of what he has called pyrog- 

 nomic minerals, such as allanite and gadolinite, which, when heated 

 to low redness, undergo a peculiar and permanent molecular change, 

 accompanied by an augmentation in density, and a change in chemi- 

 cal properties, a phenomenon completely analogous to that offered by 

 titanic acid and chromic oxyd in their change by ignition from a solu- 

 ble 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 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 



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



