244 STRUCTURE COMMON TO ALL ROCKS. 



filled in any other way. (b.) Quartz is by far the most common of all 

 vein-stuffs. Now, as already explained (p. 224), there are two varieties 

 of silica — one having a specific gravity of 2*2, the other 2'6. The dry 

 way produces only quartz-glass, which has a specific gravity of 2'2, 

 while the variety of specific gravity 2*6, or true quartz, can not be 

 formed except by the humid way.* In fact, this variety, as far as we 

 know, is always produced by slow deposition from solution. Now, the 

 quartz of veins is always the variety 2*6, and therefore was produced 

 by slow deposit from solution. The beautiful crystals so often found 

 in veins could be produced in no other way. (c.) We have already 

 seen (p. 224) that fluid cavities are a proof of formation by humid pro- 

 cess. Now, such fluid cavities are especially abundant in vein-stuffs 

 generally. They are best seen in quartz-vein stuffs, because of their 

 transparency, (d.) Not only quartz but many other minerals found 

 among vein-stuffs are of such nature that it is difficult or impossible to 

 understand how they could have been formed except by the humid 

 way, as they will not stand fusing temperature. 



2. The solutions were hot. (a.) Fissures running deep into the 

 interior of the earth could hardly remain empty of water. But from 

 their great depth the contained waters must be hot. The solvent 

 power of water, when heated to high temperature under pressure, is 

 well known. Scarcely any substance wholly resists it. (b.) The fluid 

 cavities found in quartz and other vein-stuffs are not usually entirely 

 filled, but contain a small vacuous space. Such a vacuous space indi- 

 cates (p. 225) that the inclosed liquid was at high temperature at the 

 time of being inclosed, and has since contracted on cooling. By heat- 

 ing the mineral until the cavity fills and the vacuous space disappears, 

 we ascertain the temperature of deposit. Now, by this process the 

 temperature of deposit of vein-minerals has been ascertained to vary 

 from ordinary temperatures even up to 300° and 350°. f (c.) The in- 

 variable association of metalliferous veins with metamorphism demon- 

 strates the agency of heat. 



3. TJie solutions were alkaline. — Alkaline carbonates and alkaline 

 sulphides are the only natural solvents of quartz, the commonest of 

 vein-stuffs. Moreover, when these waters contain excess of carbonic 

 acid, as is almost always the case, they dissolve also the carbonates of 

 lime, baryta, iron, etc., the next most common forms of vein-stuffs. In 

 California and Nevada such hot alkaline carbonate and alkaline sul- 

 phide springs abound, and are daily depositing silica (quartz) and car- 

 bonates of lime and of iron, and even in some cases filling fissures. 



* Recently under peculiar conditions crystallized quartz of specific gravity 2-6 has been 

 formed by dry fusion. — American Journal of Science, vol. xvi, p. 155, 1878. 

 f Sorby, Quarterly Journal of the Geological Society, vol. xiv, p. 453, et scq. 



