242 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Dec. 16, 



The following communication was read : — 



On some Peculiarities in the Microscopical Structure of 

 Crystals, applicable to the Determination of the Aqueous or 

 Igneous Origin of Minerals and Rocks. By H. C. Sorby, 

 Esq., F.R.S., F.G.S. 



[The publication of this Paper is unavoidably postponed.] 



(Abstract.) 



In this paper the author showed, that when artificial crystals are 

 examined with the microscope, it is seen that they have often caught 

 up and enclosed within their solid substance portions of the mate- 

 rial surrounding them at the time when they were being formed. 

 Thus, if they are produced by sublimation, small portions of air or 

 vapour are caught up, so as to form apparently empty cavities ; or, 

 if they are deposited from solution in water, small quantities of water 

 are. enclosed, so as to form fluid-cavities. In a similar manner, if 

 crystals are formed from a state of igneous fusion, crystallizing out 

 from a fused-stone solvent, portions of this fused stone become 

 entangled, which, on cooling, remain in a glassy condition, or become 

 stony, so as to produce what may be called glass- or stone-cavities. 



Applying the general principles resulting from his examination of 

 these phenomena to the study of natural crystalline minerals and 

 rocks, the author showed that the fluid- cavities in rock-salt, calc- 

 spar, gypsum, and some other minerals usually indicate that these 

 minerals were formed by deposition from solution in water at a tem- 

 perature not materially different from the ordinary ; also that the 

 constituent minerals of mica-schist and the associated rocks contain 

 many fluid-cavities, indicating that they have been metamorphosed 

 by the action of heated water, and not by mere dry heat and partial 

 fusion. 



From the study of the stone- and glass-cavities in slags and lavas, 

 and from the examination of the microscopic characters of quartz- 

 veins and felspar, which contain both fluid- and stone-cavities, the 

 author arrived at the conclusion that granite is not a simple igneous 

 rock, like a furnace-slag, or erupted lava, but is rather an aqueo- 

 igneovs rock, produced by the combined influence of liquid water and 

 igneous fusion, under similar physical conditions to those existing 

 far below the surface at the base of modern volcanos. 



December 16, 1857. 



Charles Wright, Esq., Wigan ; John W. Woodhall, Esq., Scar- 

 borough ; and Dr. Eugene Francfort, Clapham Road, were elected 

 Fellows. Dr. H. Abich, St. Petersburgh, was elected a Foreign 

 Member. 



The following; communications were read : — 



