Note:  Do not rely on this information. It is very old.

Mineral Veins

Mineral Veins, or Lodes, seem to have been originally fissures, the sides or cheeks of which have been coated with successive layers of minerals. These do not always completely fill the fissure; but in other cases the vein has been reopened alongside of a former infilling. Veins vary in width from less than an inch up to 150 feet or more. They occur especially among igneous and metamorphic rocks, and there is a curious relation between the nature of the country rock, as the rock traversed is termed, and the contents of the vein. Thus in Cornwall one set of parallel and contemporaneous lodes running east and west contain tin-ore where they traverse granite, and copper-ore where they are in slate ("killas"); and another set, running north and south, and of a later date, yield lead and iron ores. The minerals most commonly found in veins are the non-metalliferous vein-stones, quartz, calcite, baryte, and fluor, and the ores, such as galena, blende, cassiterite, pyrites, native copper, and gold. Though they have occasionally communicated with the surface so as to allow of pebbles and land-shells being washed down into them, veins seem generally to have been filled from below by deposits from heated solutions.