Elvan

Elvan, a Cornish term for veins of a crystalline granular mixture of quartz and orthoclase-felspar, intermediate between granite and eurite. They mostly proceed from granite bosses, penetrating both granite and surrounding slate, and frequently enclosing angular fragments of slate. They vary in width from a few inches up to 300 feet, and in Leinster they can be traced for miles in parallel bands 200 to 300 yards apart. The rock of which they are composed is termed elvanitc.