FORMATION OF VEINS. 



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Banded vein in gneis- 

 soid granite,Valparaiso. 



lateral filling of the vein by slow and successive supplies of ma- 

 terial. 



The fissures occupied by veins are simply cavities penetrating the 

 rocks more or less deeply, sometimes down to re- 

 gions of great heat, but not quite to the igneous 

 interior. During the metamorphic changes, such 

 cavities, as soon as formed, would begin to receive 

 mineral solutions or vapors from the rocks adjoin- 

 ing. The rocks could contain sufficient moisture 

 to carry on this system of infiltration, if there 

 were no other source, and the tendency of cur- 

 rents in the moisture, and any vapors present, 

 would be towards the open spaces. The mineral 

 matters thus carried to the fissure would there 

 become concreted, and commence the formation 

 of the vein. 



These materials from the adjoining rock may 

 be taken directly from it by simple solution, or be derived by a 

 decomposition of some of its constituents. And, when transferred 

 to a vein, they may be concreted unchanged, or enter into new 

 compositions through the mutual action of the several ingredients 

 there collected. 



The veins in semi-crystalline slates are mostly of quartz, because 

 silica is readily taken up by heated waters from siliceous minerals, 

 and is everywhere abundant. Many are of carbonate of lime, and 

 for a similar reason. The solutions of carbonate of lime may enter 

 from above ; but the supply has usually been derived from the ma- 

 terials of the adjoining rock through the process of infiltration. 



The veins in granitic rocks must have been often formed at the 

 high temperature required for the metamorphism of granite, and 

 the material constituting them is therefore often the same essen- 

 tially as that of the granite, only in a coarser state of crystalliza- 

 tion. 



In the infiltrating process, materials that are scattered very widely 

 and only in minute quantities through the adjoining rocks are 

 gathered gradually into these open cavities. The crystallizing of the 

 material held in solution robs the moisture of its mineral portion, 

 and will lead to a constant re-supply of it from the rock around 

 as long as the material lasts or the conditions favoring its being 

 taken up are continued. Thus veins become filled with crystals 

 of various minerals and ores that are not visible outside of them. 



The materials through every portion of a vein are not necessarily 

 derived from the rock adjoining that portion. The granitic or 



