MINERAL VEINS. 733 



material lasts, or the conditions favoring its being taken up are con- 

 tinued. Thus veins become filled with crystals of various minerals 

 and with ores that are not visible in the rock outside of them. 



The minerals through any particular portions of a vein are not 

 necessarily derived from the rock adjoining that portion. The granitic 

 or other material derived from its deeper part may rise and occupy the 

 vein where it intersects slate-rocks. 



With this mode of filling, when the process is very slow, the outer 

 layers, or those lying against the inclosing walls, will be first formed, 

 and then another layer inside of this, and so on, until the whole, to 

 the centre, is occupied. By such means, the banded structure is pro- 

 duced. Owing to the varying circumstances, during the slow filling 

 of a vein, — the work sometimes evidently of a long period, — the 

 infiltrating material varies in kind ; and hence comes the variation in 

 the minerals constituting the successive layers. Some of the layers, 

 especially the metallic, may be formed from vapors or solutions rising 

 from a deeper source than the range of level along which they occur. 



If the process of filling were rapid, the vein would fail of this 

 division into layers. The adjoining rock is often cotemporaneously 

 altered. 



Certain veins in crystalline rocks, which blend on either side with the rock adjoining, 

 are sometimes called segregated veins. They are supposed to have been formed by a 

 segregating process, or a crystallization out of the rock in which they occur, the direc- 

 tion of the plane of the vein being determined, not by the previous existence of a fis- 

 sure, but by magnetic currents through the rock, or other less intelligible cause. No 

 facts authorize us to infer that magnetic currents have the power here attributed to 

 them. Such a blending of a vein with the walls is a natural result, when its formation 

 in a fissure takes place at a high temperature during the metamorphism or crystalliza- 

 tion of the containing rock. 



3. Alterations of Veins. — Veins do not always retain their original 

 constitution ; and those that are metalliferous are especially liable to 

 alteration. There are often lines of small cavities through the middle 

 of a vein, or along its sides, or in both ; and, when the rocks in which 

 they occur are raised above the level of the ocean, the atmospheric 

 waters find access as they become subterranean, and constantly trickle 

 through them. These waters decompose some species readily (pyrite, 

 etc.), and take the new ingredients (sulphate of iron, etc.) into solu- 

 tion. Feldspathic minerals may be decomposed, and the waters thereby 

 become siliceous and alkaline. Also, in one way or another, they may 

 become carbonated. Thus armed, the waters go on making various 

 changes in the ores and minerals of the vein, altering chalcopyrite 

 (sulphid of copper and iron) to copper-glance or erubescite (sulphids 

 of copper), or to malachite (carbonate of copper), or changing in a 

 similar manner ores of silver or lead, etc. In some parts, the arrange- 



