MINERAL VEINS AND ORE-DEPOSITS. 



773 



ranged in layers parallel to the walls. Banded veins are much the 

 less common kind. Frequently, only the broad parts or enlargements 

 of a vein are banded and metalliferous. A banded vein of the sim- 

 plest kind consists of a band of ore and one band either side of 

 rock-material. But there are at times several bands of each. These 

 bands may be alike on the opposite sides of the middle, except re- 

 versed in order, with variations, but chiefly in thickness ; or they may 

 be wholly unlike ; and they sometimes indicate that the vein is in 

 places or wholly a combination of two or more veins. In Fig. 1130, 

 representing a portion of a Cornwall vein, near Reduth (from De la 



Fig. 1130. 



Fig. 1131. 



m 



Sections of Cornwall Veins. 







1 



m 



wPMziyim 



miasm 



a 



Beche), the vein is made up of six quartz veins, each crystallized, and 

 bearing, with one exception, some other mineral along the centre ; a 

 and d containing some fluorite ; b and f a little, and c much, chalco- 

 pyrite, or copper pyrites. Fig. 1 131 represents a section of another 

 Cornwall lode, at Godolphin Bridge ; it consists of either two or three 

 veins side by side : a, consisting of quartz ; b c b, made up of agate 

 in bands along the sides and quartz at the centre ; and d, a band of 

 chalcopyrite or copper pyrites. Again, in Fig. 133, on page 112, 

 there are three combined veins : d d, one ; C, a second, consisting of 

 ore ; and a bb c, a third. 



The rock-material of the vein may be granite, syenyte, or some re- 

 lated kind of rock, or it may be quartz, or spars. The most common of 

 the spars in veins are calcite, barite (heavy spar or barium sulphate), 

 and fluorite (fluor spar, calcium fluoride). The band of ore may be an 

 even, continuous one; but the ore is oftener involved in the spars or 

 quartz, occurring in large masses at one place, and only in threads or 

 in scattered grains or crystals, or absent wholly, in another. Wide 

 openings often occur along the centre of a vein, containing one or 

 many minerals together. 



True veins, as those of granite, quartz, or other material, which in- 

 tersect the rocks deeply, have never a transversely columnar structure 

 as a result of contraction or cooling, and in this respect are unlike 



