190 T. S. Huni^Notes on Granitic Bocks. 



silicious veinstones now in process of formation in open :: 

 in Nevada. (L. E. and D. Phil. Mag. (4), xxxvi, 821, 422, a:. 

 Jour., II, xlvii, 138). We cannot doubt that the ancient, likt - 

 modern veins, have been channels for the discharge of subtcr 

 ranean mineral waters, and it would seem that while the dep»> 

 sition of the incrusting materials on the walls of the fissure b 

 in part due to cooling, and in part perhaps to the infiltraticr.. 

 some cases, of precipitants from lateral sources, it is cli; '' ' 

 be ascribed to the reduction of solvent power consequeir 

 the diminution of pressure as the waters rise nearer to t^ 

 face. This conclusion, deducible from the researches ol - 

 on the relation of pressure to solubility, I have pointed 

 the Geological Magazine for February, 1868, p. 57. ^ 

 this Journal, II, 1, 27. 



§29. There is evidently a distinction to be drawn l>-' 

 veins which have been open channels and the segregated i: 

 and geodes formed in cavities which appear to have been i \ i , 

 where limited by the enclosing rock. In the former case, a t;|- 

 circulation of the mineral solution would prevail, while m tlie 

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

 diffusion through the r6ck. A comparison between the coiitt • ^ 

 of geodes and fissure- veins, whether in granitic rocks or lu 

 iferous limestones, will however show that these differei 

 not sensibly affect the mineral constitution of the depos :■ 



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

 species may be formed is apparently very great. Sorby, ^ , 

 his investigations of the fluid-cavities of crystals, conau; - 

 ^^ ' ■■■ . ^ . ^ ■ land feW" 



3 granitic veins of Cornwall, must have crystallized 

 peratures from 200° to 340° Centigrade, and under great pj^ 

 sure, conditions which we can hardly suppose to have presw^ 

 over the production of the crystallized quartz found iQ*^®, 

 tered tertiaries of the Paris basin, or the auriferous congionj^^ 

 rates of California. In like manner beryl, though a com 

 mineral of the tin-bearing granite veins, like those stjoieu . 

 Sorby, occurs at the famous emerald mine of Muso m ^^^ ,. 

 nada, in veins in a black bituminous limestone, holding ^^,,^ 

 nites, and of Neocomian age, its accompaniments being c^^^. 

 quartz and carbonate of lanthanum (parisite). Small crj^ ^ 

 of emerald are disseminated through this argillaceous^ ^^ 

 what magnesian limestone, which contains moreover a ^^^ 

 amount of glucina in a condition soluble in acid& i^^'lpxi 

 deCh. et Phys., liii, 1-26, and Foumet, Geol. Lymnaise, 4^^ 

 1 31. To these we may add the production of vanous ^^^ 

 ted crystallized silicates, including apophyllite, harmotom ^^^ 

 chabazite, during the historic period in the masonry of j^e f 

 Roman baths at Plombi^res and Luxeuil, and by the actio 



