MINERAL VEINS. 235 



squeezing and crushing. The small fissures thus produced are often 

 filled by lateral secretion from the walls, or else by slowly-percolating 

 waters holding in solution the more soluble matters contained in the 

 rocks. The process is similar to the filling of cavities left by imbedded 

 organisms (p. 193), and still more to the filling of vapor-blebs in traps 

 and lavas, and the formation of agates and carnelian amygdules (p. 

 215). In veins of this kind, therefore, a beautiful ribbon-structure is 

 often produced by the successive deposition of different-colored mate- 

 rials on the walls of the fissure. Veins of this kind also, since they 

 are the filling of a previously-existing fissure, have distinct walls. The 

 filling consists most commonly of silica or of carbonate of lime. G-ash- 

 veins of authors are probably larger veins of this kind. 



3. Fissure- Veins. — These are fillings of the great fissures produced 

 by movements of the earth's crust. When these fissures are filled at 

 the time of formation by igneous injection, they are called dikes ; but 

 if subsequently with mineral matter, by a different process, to be dis- 

 cussed hereafter, they are fissure-veins. These veins, therefore, like 

 dikes, outcrop over the surface of the country often for many miles, 

 fifty or more. Like dikes, also, they are often many yards in width, and 

 extend to unknown, but certainly very great, depths. Like dikes and 

 fissures, also, they occur in parallel systems. 



Characteristics. — The most obvious characteristics of the veins of 

 this class are their size, their continuity for great distances and to 

 great depths, and their occurrence in parallel systems. As the vein is 

 a filling of a previously-existing fissure, the distinction between the 

 vein and the wall-rock is usually quite marked. In many cases, in 

 fact, the vein-filling is separated from the wall-rock by a layer of tena- 

 cious, clayey matter called a selvage. The selvage is probably formed 

 by decomposition of the wall-rock in immediate contact with the vein, 

 by circulating water. The contents of fissure-veins are also far more 

 varied than those of other classes. 



Metalliferous Veins. — Some metals, particularly iron, occur prin- 

 cipally in great beds, being accumulated by a process already described 

 (p. 144). Others, especially lead, often accumulate in flat cavities be- 

 tween the strata, especially of limestone. But most metals occur in 

 veins. All the kinds of veins mentioned above may contain metals, 

 but the segregative veins are usually too irregular and uncertain, and 

 the infiltrative veins too small, to be profitable. True, profitable metal- 

 liferous veins are almost always great fissure-veins. We will speak, 

 therefore, principally of these, and the further description of fissure- 

 veins is best undertaken under this head. 



Contents. — The contents of metalliferous veins are of two general 

 kinds, viz., vein-stuffs and ores. The principal vein-stuffs are quartz, 

 carbonate of lime (calc-spar), carbonate of baryta, carbonate of iron, 



