ON THE FORMATION OF CRYSTALLINE ROCKS. 24J> 



been produced b_y a purely igneous fusion. Breislack,* Fuchs, and 

 Boucheporn, Schafhaiitl and Scheerer,t concluded from the abundance 

 of isolated grains of quartz, from the manner in Avhich its elements 

 are grouped together, in fine, from the presence of pyrognomic mine- 

 rals sometimes found in it, that granite originally contained water, 

 and, moreover, that the presence of this water could have prolonged 

 the plasticity of the mass much below its point of actual fusion. 

 Elie de Beaumont showed, in addition, that granite probably owe& 

 its mineralogical composition to different substances, which, since its 

 consolidation, have partly disappeared with the water, such as com- 

 pounds of chlorine, fluorine, and boron. Thus the mode of formation 

 of granite takes a character intermediate between the origin of me- 

 tallic veins and that of volcanic and basic eruptions, J and the emi- 

 nently crystalline condition of this rock is not caused by its having 

 been solidified at great depths. 



The ingenious observations of Sorby on the liquids enclosed in 

 the microscopic vacuities of rocks have entirely confirmed the inter- 

 vention of water and heat in the formation of granite. § It will be 

 remembered that the attention of Davy had been already arrested by 

 the minute drops of liquids, accompanied by gaseous matters, v,diich 

 may be distinguished by the naked eye in certain crystals, and that 

 their examination had led him to suspect that water, with the aid of 

 pressure, had contributed to the formation of rock crystal. Later^ 

 Sir David Brewster has given greater extent to this species of 

 researches.il 



Another order of facts which has still further confirmed the induc- 

 tion that granite might have been plastic without the intervention of 

 a very high temperature is, that the rocks into which it has been in- 



■~ Breislack, although approving with energy the ideas of the neptunists, remarks, that 

 from the order of consolidation of the elements of granite, and the presence of drops of 

 liquid which quartz sometimes contains, it is difficult to admit that this rock has ever been 

 in a real state of fusion. " Why could not water and fire have co-operated together at 

 different times in the production of our earth, and sometimes even have joined their 

 efforts?" — Instituiions Geologiques, (French translation,) vol. i, p. 68, 1818. 



Thus he sought to follow out the idea of which Faujas-Saint-Fond and Spallanzani had 

 already caught a glimpse, in bringing together two agents which are in no way incompati- 

 ble, as his two antagonists pretended, and which he, on the contrary, saw intimately as- 

 sociated in the volcanoes of which he had made a profound study. "If," says Breislack, 

 "experiments ought to serve as a guide to philosophy, and if those which are carried on 

 in volcanoes are the most importiint which we can collect, why may we not use them ?"' 



t Discussion sur la nature plutonique du granite et des silicates cristallins qui s'y ral- 

 lient. — (Bulletin de la SocietU Gtologique de France, 2d8eries, vol. iv, p. 468. Reponse aux 

 objections de Durocher, Idem, vol. vi, p. 644, and vol. viii, p. 500.) 



J Note sur les emanations volcaniques. — [Bulletin de la Socieli6 Geologique de France, 2d 

 series, vol. iv, p. 1291.) Eliede Beaumont describes the plastic state of granite under the 

 nRine of gelatinous surf usion, p. 1310. 



§ Sorby on the microscopical structure of crystals, &c., Quarterly Journal of the Geol-ogi- 

 eal Society, p. 453, 1858. — (CompUs rendtts de l' Academic, vol. xlvi, p. 46.) 



Ij Annales de Chimie et de Physique, vol. xxi, p. 182, 1822. 



The topaz of Brazil, according to Sir D.tvid Brewster, also contains different liquids. — 

 [Poggtrulorff' s Annalen, vol. vii, p. 493.) 



