658 THE POPULAR SCIENCE MONTHLY. 



sionally 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, occa- 

 sionally 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 daz- 

 zling white feldspar. One is constantly tempted to fill every 

 available pocket with the mineral, to the exclusion of other speci- 

 mens really more interesting. Interspersed with the feldspar are 

 masses of grayish-white quartz and occasional blocks of the cov- 

 eted mica. 



It would be of the highest value to know how these three min- 

 erals 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 in- 

 truded 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 mate- 

 rial came into the vein in a condition of aqueo-igneous fusion 

 that is to say, rendered liquid at a comparatively low tempera- 

 ture 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 cir- 

 cumstantial 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 con- 



