266 MINERAL VEINS 



banded. In the former case the vein is filled with a single min- 

 eral, while in the latter mineral substances are arranged in bands, 

 which are, in general, parallel to the walls of the fissure. These 

 bands were evidently deposited on the walls of the fissure, for the 

 more perfect ends of the crystals project inward toward the mid- 

 dle line of the vein, and the bands are arranged in corresponding 

 pairs, from the walls inwards. In many instances an apparent 

 departure from this symmetrical arrangement is produced by a 

 reopening of the fissure, so that the older vein forms one of the 

 w T alls of the new one. Frequently the minerals are associated with 

 an eruption of igneous rock, and the minerals are deposited along 

 the plane of contact between the igneous mass and the country 

 rock. 



Often mineral veins contain in greater or less richness the ores 

 of various metals, and then they are called metalliferous veins. 

 From such are obtained the greater part of the world's supply 

 of gold, silver, copper, tin, etc. The minerals which form the 

 larger bulk of the vein are called vein stuff, or gangue, and the ores 

 are either gathered in threads, pockets, or nuggets, or dissemi- 

 nated in fine grains throughout the mass of the vein stuff. The 

 principal minerals which make up the latter are quartz, calcite, 

 barite (heavy spar, BaS0 4 ), and fluor-spar, while the metals are 

 sometimes native {i.e. in the free, uncompounded state), as are 

 gold and platinum nearly always, and copper frequently, but they 

 much more commonly occur as sulphides, chlorides, oxides, car- 

 bonates, or other combination. 



The outcrop of a mineral vein at the surface of the ground is 

 much altered by weathering and the true character of the vein 

 may appear only after it has been worked. The chemical changes 

 produced by weathering vary, of course, with the nature of the 

 materials acted on, and a single example must suffice here. In 

 the deeper, unaltered portions of many gold-bearing veins the 

 gold is contained in crystals of iron pyrites. For a varying depth 

 below the surface, the gold is native and is scattered in minute 

 threads and grains through a mass of broken quartz, stained rusty 

 red or brown by iron, while the pyrite has disappeared. The 



