Ch. TI.] OEIGIN OF MINERAL VEINS. 97 



tortuous dykes of diabase ; 3rdly, Elvan dykes ; and, 

 lastly, auriferous quartz veins. In every region of 

 intrusive plutonic rocks that has been thoroughly 

 explored, a succession of events, culminating in the pro- 

 duction of mineral veins, has beea proved to have taken 

 place,* and we are bound to look upon the origin of such 

 veins as the natural result of the plutonic intrusion ; 

 there is, also, sometimes a complete gradation from 

 veins of perfectly crystallised granite, through others 

 abounding in quartz at the expense of the other con- 

 stituents up to veins filled with pure quartz, as at Porth 

 Just, near Cape Cornwall ; and, again, the same vein 

 will in some parts be filled with felspar ; in others, con- 

 tain irregular masses of quartz, apparently the excess of 

 silica beyond what has been absorbed in the trisilicate 

 compound of felspar, f Granitic, porphyritic, and trap- 

 dykes J also sometimes contain gold and other metals ; 

 and I think the probability is great that quartz veins 

 have been filled in the same manner, that if dykes and 

 veins of granite have been an igneous injection, so have 

 those of quartz. By an igneous injection, I do not 

 mean that the fused rock owed its fluidity to dry heat. 

 The celebrated researches of Sorby on the microscopical 

 fluid cavities in the quartz of granite and quartz veins, 

 have shown beyond a doubt that the vapour of water 

 was present in comparatively large quantities when the 

 quartz was solidifying. All strata below the surface 

 contain water, and if melted up would still hold it as 



* " Mineral Veins," p. 1C. 



f Mr. John Phillips in " Memoirs, Geological Survey of Great 

 Britain," Vol. ii. p. 45. 



J Sir R I. Murchison, " Siluria," pp. 479, 481, 488, and 500 ; and 

 R. Daiutree, Quart. Jour. Geol. Soc., vol. xxviii. pp. 308,310. 



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