732 DYNAMICAL GEOLOGY. 



sometimes to considerable distances, either diffusing ores through them, 

 or filling cracks or long fissures. 



Second Method. — The second method is exemplified only in super- 

 ficial veins, seams, or cavities. Carbonate of lime is often thus de- 

 posited in seams or open cavities. 



Third Method. — The third method is that by which the great ma- 

 jority of the veins in metamorphic rocks, whether simply stony or 

 metalliferous, were produced. The nature of the minerals constituting 

 veins, their associations, and the banded structure often characterizing 

 them, are opposed to their formation by injection. Examples of the 

 banded structure are represented in Figs. 132, 133, p. 112. Such a 

 banded arrangement could have resulted only from a lateral filling of 

 the vein, by slow and successive supplies of material. 



The fissures occupied by veins are simply cavities penetrating the 

 rocks more or less deeply, sometimes down to regions of great heat, 

 but not to those of fused rock. During the metamorphic changes, 

 such cavities, as soon as formed, would begin to receive mineral solu- 

 tions or vapors from the rocks adjoining. The rocks may contain suf- 

 ficient moisture to carry on this system of infiltration, if there were 

 no other source ; and this moisture, and any vapors present, would 

 move toward the open spaces. The mineral matters thus carried to 

 the fissure woidd 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 materials of 

 the adjoining rock, through the process of infiltration. 



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

 temperature required for the metamorphism of granite ; and the ma- 

 terial constituting them is therefore often the same essentially as that 

 of the granite, only in a coarser state of crystallization. 



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, so long as the 



