Page:Popular Science Monthly Volume 41.djvu/676

658 it changes abruptly and cuts across the strata. In some of the mines the vein does not come to grass, as the miners say, but only begins some distance below the surface. The veins vary in thickness from less than an inch to ten or a dozen feet, occasionally to as much as thirty or forty feet, but these instances are rare. In places the vein pinches out completely and is practically lost, or is cut off perhaps by a large mass of displaced country rock, known as a "horse."

The contrast between the vein stuff and its containing walls is very striking and often very beautiful. The "slate" is almost black, and is generally clean and glistening, while the vein itself is almost snow-white. This is due to the feldspar with which the fissure is filled. It breaks with a clean, smooth cleavage, and shows on such surfaces a brilliant, pearly luster. The dump-heaps around the mine-mouth are largely made up of this dazzling white feldspar. One is constantly tempted to fill every available pocket with the mineral, to the exclusion of other specimens really more interesting. Interspersed with the feldspar are masses of grayish-white quartz and occasional blocks of the coveted mica.

It would be of the highest value to know how these three minerals got into the vein and arranged themselves in their present form, but, as no direct observation is possible, we can only reason back from such facts as we are now able to observe. The fissures themselves are doubtless simple cracks formed by those shiftings and readjustments which are constantly going on in the surface rocks of the earth. The vein material has evidently been intruded from below and has come in a liquid or pasty condition, but just how it has come, and whether as a uniform mass which afterward separated into the different minerals, or as a mixture in which each mineral still preserved its own identity, we are quite unable to say. The most reasonable supposition is that the material came into the vein in a condition of aqueo-igneous fusion—that is to say, rendered liquid at a comparatively low temperature by the presence of water and great pressure—and that it was fairly homogeneous. The question as to which mineral separated first would seem almost hopeless. Yet there is quite strong circumstantial evidence to show that the mica was the first to form, for the mica is much more uniformly crystallized than either of the other two minerals, and frequently leaves the impress of its lamina on the crystals of quartz. After the mica, the feldspar probably separated; and, last of all, the silica that was left over after the formation of these two minerals, collected into crystals of quartz. This is what we would expect theoretically. The mica is only about half silica, the feldspar a little over two thirds, and the quartz manifestly nearly all silica. The minerals