XL] KANITES AND GKANITIC VEIN-STONES. 205 



masses and geodes formed in cavities which appear to have 

 been everywhere 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 be- 

 tween the contents of geodes and fissure-veins, whether in 

 granitic rocks or in fossiliferous limestones, will however show 

 that these differences do not sensibly affect the mineral consti- 

 tution 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 feld- 

 spar in the granitic veins of Cornwall must have crystallized 

 at temperatures from 200 to 340 Centigrade, . and under great 

 pressure ; conditions which we can hardly suppose to have pre- 

 sided 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 neocomian age, its accompaniments 

 being calcite, quartz, and carbonate of lanthanum (parisite). 

 Small crystals of emerald are disseminated through this argil- 

 laceous, somewhat magnesian limestone, which contains, more- 

 over, a small amount of glucina in a condition soluble in acids. 

 (Lewy, Annales de Chimie et de Physique, LIII. 1 - 26 ; and 

 Fournet, Geol. Lyonnaise, 455.) 



31. To these we may add the production of various hy- 

 drated crystallized silicates, including apophyllite, harmotome 

 and chabazite, during the historic period in the masonry of the 

 old Eoman baths at Plombieres and Luxeuil, and by the ac- 

 tion of waters at temperatures of from 46 to 70 Centigrade 

 (ante, page 25) ; the presence of apophyllite, natrolite, and stil- 

 bite in the lacustrine tertiary limestones of Auvergne ; apophyl- 

 lite incrusting fossil wood, and chabazite crystals lining shells in 



