HEAT — VEINS. 



383 



or more sets. Eig. 307 appears as if made up of two veins side by side, ahcb 

 one, and d another; two bands h are agate bands (uncrystallized quartz), and 

 at c are two bands of crystallized quartz. The two sides of the fissure received 

 simultaneously the deposition of agate, and then, over this, the layer of quartz 

 in crystals. If a band or string of ore had been deposited between the two 

 of quartz, as is common, this would have made it an ore-vein. But in the 



309 



Compound veins from Cornwall. De la B. 



figure, the large band d is ore, copper ore ; and to make it, the fissure was 

 reopened along the wall to the left, and the ore introduced without any 

 " gangue " material. Fig. 308 represents a triple vein, abha one, c a second, 

 and dd the third ; and Fig. 309, a sextuple vein, or one that was opened six 

 times for new vein-making. Each of the six parts is called a comb in 

 miners' language. In one great vein, opened at Freiburg, the layer consisted 

 of blende (ZnS), quartz, fluorite (CaF), pyrite, galena, barite, calcite, each two 

 or three times repeated, the layers nearly corresponding on either side of the 

 middle seam. 



The ore of veins occurs in one or several of the bands ; or is gathered 

 along the center ; or collected in the broader portions or swellings of a vein, 

 making nests ; or distributed through the gangue. 



Most quartz veins cutting through crystalline rocks are actually simple, 

 though begun in each case by deposition against the walls. Gold-bearing 

 veins are commonly ordinary quartz veins, but the gold is usually in 

 minute, invisible scales through the quartz, though occasionally in threads 

 of crystals, and "nuggets " or larger masses. In the case of the gold-bearing 

 quartz, crushing, and then either washing or amalgamation, are required to 

 obtain the gold. Gold-bearing quartz veins contain also more or less pyrite 

 in which gold is often present profitably, and also often galena (PbS) and 

 sphalerite or blende (ZnS). A region of chloritic or hydromica schist 

 having interlaminating and intersecting veins of quartz, in which occur 

 some pyrite and galena, is almost always a gold region. 



The banded structure of many veins is one of the points in which veins 

 differ from dikes. But they are often like dikes in having contact minerals 

 in the walls of the veins, due to the same process which filled the vein. 



