454 Transactions. — Geology. 



When granite and other rocks have penetrated superincumbent strata, 

 the contact of the molten mass has usually produced a change in the neigh- 

 bouring rocks, but of a different character to that widespread uniform 

 character described under metamorphism. The difference has been brought 

 about through its sudden character, and probably by the loss of a large 

 portion of watery vapour, causing a vitrifying effect to be produced. 

 Hornblende-slates are frequently formed along the contact margins of 

 granite and clay-slates. (Q.J.Gr.S., vol. xxxii., p. 187, J. A. Phillips; 

 Q.J.G.S., vol. xxxiv., p. 438). 



Metalliferous veins are usually found to have been formed by the 

 occurrence of intrusive rocks in the vicinity, the latter having been usually 

 decomposed by acids and vapours, the introduced metals, or the metals 

 from them, being deposited in veins. 



Mr. J. A. Phillips in Q.J.G.S., vol. xxxv., p. 391, describes the dis- 

 trict of Steamboat Springs, Nevada, where " fissures are being lined with 

 siliceous incrustations which are being constantly deposited, while a central 

 longitudinal opening allows the escape of gases, steam and boiling water ; 

 the water is slightly alkaline and contains carbonate of sodium, sulphate of 

 sodium, common salt, etc." These springs have deposited cinnabar 

 (ore of mercury) with the silica (both amorphous and crystalline, the latter 

 containing the usual liquid cavities and ordinary optical and other charac- 

 ters of ordinary quartz). At other springs in the same district silver and 

 gold have been found enclosed in sinter-like deposits. In Australia gold 

 occurs in pyrites contained in diorites and granite, and gold mines are 

 worked in these rocks. Mineral veins often show by their structure that 

 the fissures (Q.J.Gr.S., vol. xxxii., p. 169) they fill have been widened re- 

 peatedly, probably by the force of crystallization, successive infiltration 

 having filled the fissure with siliceous and other substances forming a banded 

 structure. The metals when they occur may either have been deposited 

 from solution or by sublimation. The tin-bearing bands of schorl rock in 

 granite of Cornwall, have been proved to have been formed through the 

 decomposition of the granite along the sides of leaders or veins. Granites 

 vary from coarsely porphyritic granites to the fine grained elvans (quartzi- 

 ferous porphyry) in which mica is present. The porphyritic texture is due 

 to the inequality in the crystallizing power of the various minerals, felspar 

 and mica crystallizing more readily than quartz, the latter always occurring 

 in consequence in more irregular forms than the former. 



Hornblende and schorl are sometimes found replacing mica to a great 

 extent, forming syenite and schorlaceous-granite, and when only a small 

 proportion of quartz occurs the rock passes into syenite and schorl- 

 rock. Granite frequently passes into felstone, micaceous lelstone differing 



