1870.] T. S. HUNT — ON GRANITIC ROCKS. 405 



now in process of formation in open fissures in Nevada (L. E. and 

 D. Pliil. Mag. (4), xxxvi, 321, 422, Amer. Jour, of Science II, 

 xlvii, 138). We cannot doubt that the ancient, like these modern 

 veins, have been channels for the discharge of subterranean mineral 

 waters, and it would seem that while the deposition of the incrust- 

 inn: materials on the walls of the fissure is in part due to coolin"-. 

 and in part perhaps to the infiltration, in some cases, of precipitants 

 from lateral sources, it is chiefly to be ascribed to the reduction of 

 solvent power consequent upon the diminution of pressure as the 

 waters rise nearer to the surface. This conclusion, deducible from 

 the researches of Sorby on the relation of pressure to solubility, I 

 have pointed out in the Geological Magazine for February, 1868, 

 p. 57. See also Amer. Jour, of Science, II, 1, 27. 



§ 29. There is evidently a distinction to be drawn between 

 veins which have been open channels, and the segregated masses 

 and geodes formed in cavities which appear to have been every- 

 where limited by the enclosing rock. In the former case, a free 

 circulation of the mineral solution would prevail, while in the latter 

 there could be no renewal of it except by percolation or diffusion 

 through the rock. A comparison between the contents of geodes 

 and fissure-veins, whether in granite rocks or in fossiliferous lime- 

 stones, will however show that these differences do not sensibly 

 affect the mineral constitution of the deposits. 



§ 30. The range of conditions under which the same mineral 

 species may be formed is apparently very great. Sorby, from his 

 investigations of the fluid-cavities of crystals, concludes that the 

 quartz which occurs with cassiterite, mica and feldspar in the 

 granitic veins of Cornwall, must have crystallized at temperatures 

 from 200° to 340° Centigrade, and under great pressure, con- 

 ditions which we can hardly suppose to have presided over the 

 production of the crystallized quartz found in the unaltered 

 tertiaries of the Paris basin, or the auriferous conglomerates of 

 California. In like manner beryl, though a common mineral of 

 the tin-bearing granite veins, like those studied by Sorby, occurs 

 at the famous emerald mine of Muso in New Grenada, in veins in 

 a black bituminous limestone, holding ammonites, and of neoco- 

 mian age, its accompaniments being calcite, quartz and carbonate 

 of lanthanum (parisite). Small crystals of emerald are dissemi- 

 nated through this aigillaceous somewhat magnesian limestone 

 which contains moreover a small amount of glucira in a condition 



