488 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



included stone, explains why there are no vacuities in the stone- 

 cavities of granites and elvans. In the quartz of very coarse-grained 

 granites the stone-cavities are generally obscure and of irregular 

 shape, as shown by fig. 118. Those in the felspar are often so much 

 obscured by the partial decomposition of that mineral, that it is 

 difficult to distinguish them from small decomposed patches ; but in 

 some very clear from the granite of Lamorna they are sufficiently 

 distinct, and, as shown by fig. 120, are very analogous to those in 

 the felspar of the trachyte of Ponza, figs. 71 and 72. 



Besides fluid- and stone-cavities, the quartz of granite often con- 

 tains vapour-cavities, like those in minerals from modern volcanos. 

 Some are almost perfect spheres, and exactly like enclosed bubbles 

 of gas ; but others are of more irregular shape, and gradually inter- 

 fere with and pass into fluid-cavities, in the same manner as occurs 

 in some of the minerals of ejected blocks, and in crystals formed ar- 

 tificially by alternate exposure to liquid and the air. Some of these 

 empty cavities may be fluid- cavities that have lost their fluid, but I 

 have found them in specimens obtained on the sea-coast below low- 

 water mark, which were afterwards kept under water and never 

 dried, and therefore some must certainly be genuine gas- or vapour- 

 cavities. 



On the whole, then, the microscopical structure of the constituent 

 minerals of granite is in every respect analogous to that of those 

 formed at great depths and ejected from modern volcanos, or that 

 of the quartz in the trachyte of Ponza, as though granite had been 

 formed under similar physical conditions, combining at once both 

 igneous fusion, aqueous solution, and gaseous sublimation. The 

 proof of the operation of water is quite as strong as of that of heat ; 

 and, in fact, I must admit, that in the case of coarse-grained, highly 

 quartzose granites there is so very little evidence of igneous fusion, 

 and such overwhelming proof of the action of water, that it is im- 

 possible to draw a line between them and those veins where, in all 

 probability, mica, felspar, and quartz have been deposited from solu- 

 tion in water, without there being any definite genuine igneous fusion 

 like that in the case of furnace slags or erupted lavas. There is, 

 therefore, in the microscopical structure a most complete and gradual 

 passage from granite to simple quartz- veins ; and my own observa- 

 tions in the field cause me to entirely agree with M. Elie de Beau- 

 mont (Note sur les emanations volcaniques et metalliferes, Bulletin 

 de la Societe Geologique de France, 2 serie, t. iv. p. 1249) in con- 

 cluding that there is also the same gradual passage on a large scale. 



My remarks respecting the possibility of the water having passed 

 into the fluid-cavities in nepheline after they were formed, will, to a 

 considerable extent, apply to the fluid-cavities in the quartz of elvans 

 and granites. If they had contained nearly pure water, and were 

 quite full, and easily lost it on drying, such a supposition would have 

 been sufficiently probable. It is, however, not mere water, but va- 

 rious saline solutions, with, free acids, precisely like the fluid in the 

 cavities of some modern volcanic minerals. Moreover, that a very 

 great pressure will not cause water to pass through solid crystal, ex- 



