The extensive mining operations to which veins have been subjected in all parts of the world, have made our knowledge of their forms below the surface very full and accurate. It has been learned in this way that very often the corresponding portions of the walls of a vein do not coincide in position, but one side is higher or lower than the other, showing that the walls slipped over each other when the fissure was formed or subsequently; and this faulting or displacement of the walls appears to be much more common with veins than with dikes, perhaps because the fissures remained open much longer. This slipping of the walls is the principal cause of the almost constant changes in the width of veins. For, since the walls are never true planes, and are often highly irregular any unequal movements must bring them nearer together at some points than at others. As a rule, the enormous friction accompanying the faulting, either crushes the wall-rock, or polishes and striates it, producing the highly characteristic surfaces known as _slicken-sides_. Where the wall is finely pulverized in this way, or is partially decomposed before or after the filling of the fissure, a thin layer of soft, argillaceous material is formed, separating the vein proper from the wall-rock. The miners call this the _selvage_; and it is a very characteristic feature of the true fissure veins. Fragments of the wall-rock are frequently enclosed in veins, and the latter sometimes branch or divide in such a way as to surround a large mass of the wall, which is known as a “horse.” A similar result is accomplished when a fissure is re-opened after being filled, if the new fissure does not coincide exactly with the old. It has been proved that veins have thus been re-opened and filled several times in succession; and in this way fragments of the older vein material become enclosed in the newer. Although usually determined in direction by the joint-structure of the country rock, veins are often parallel with the bedding, especially in highly inclined, schistose formations. Such interbedded veins are commonly distinctly lenticular in form, occupying rifts in the strata which thin out in all directions and are often very limited in extent. Whether conforming with the joint-structure or bedding, veins are commonly arranged in systems by their parallelism, those of different systems or directions usually differing in age and composition, and the older veins being generally faulted or displaced when intersected by the newer.