714 DYNAMICAL GEOLOGY 



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 enclosing 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 

 produced. 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 varia- 

 tion in the minerals constituting the successive layers, as described 

 on page 123. 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. 



Thus, quartz may be succeeded by fluor spar, and this by an ore 

 of one or more metals ; the last by quartz again, or by calcite ; and 

 so on in various alternations. 



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 ad- 

 joining 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 direction of the plane of the vein being determined, not by the 

 previous existence of a fissure, but by magnetic currents through the rock, or 

 other less intelligible cause. No facts authorize us to infer that magnetic cur- 

 rents 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 crystallization 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 at- 

 mospheric waters find access as they become subterranean, and 

 constantly trickle through them. These waters decompose some 

 species readily (iron pyrites, etc.), and take the new ingredients 

 (sulphate of iron, etc.) into solution. Feldspathic minerals may be 

 decomposed, and the waters thereby become siliceous and alkaline ; 

 or, in one way or another, they may btvcome carbonated. Thus 

 armed, the waters go on making various changes in the ores and 

 minerals of the vein, altering copper pyrites (sulphuret of copper 

 and iron) to copper-glance or erubescite (sulphurets of copper), or 

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

 ores of silver, or lead, etc. In some parts, the arrangements may 



