Origin of Mineral Veins iy 
plete gradation from veins of perfectly crystallised granite, 
through others abounding in quartz at the expense of the 
other constituents, 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, contain 
irregular masses of quartz, apparently the excess of silica 
beyond what has been absorbed in the trisilicate compound 
of felspar.1 Granitic, porphyritic, and trappean dykes 2 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 super- 
heated steam; and M. Angelot has suggested that fused rock 
under great pressure may dissolve large quantities of the 
vapour of water, just as liquids dissolve gases. The presence 
of the vapour of water would cause the liquefaction of quartz 
at a much lower temperature than would be possible by heat 
alone, unaided by water.* I know that this opinion is con- 
trary to that usually held by geologists, the theory generally 
accepted being that mineral veins have been produced by 
deposits from hot springs; but during twenty years I have 
been engaged in auriferous quartz-mining in various parts of 
the world, and nowhere have I met with lodes, the pheno- 
mena of which could be explained on this hypothesis. The 
veinstone is pure quartz containing water in microscopical 
cavities, as in the quartz crystals of granite, but not com- 
bined as in the hydrous siliceous sinter deposited from hot 
springs. The lodes are not ribboned, but consist of quartz, 
jointed across from side to side, exactly like trappean dykes. 
There is often a banded arrangement produced by the re- 
? Mr. John Phillips in Memoirs, Geological Survey of Great Britain, 
vol. ii. p. 45. 
4Sir R. I. Murchison, Siluria, pp. 479, 481, 488, and 500; and 
R. Daintree, Quart. Journ. Geol. Soc., vol. xxviii. pp. 308, 310. 
3H. C. Sorby, Journ. Geol. Soc., vol. xiv. 
