horse
A mass of earthy matter, or rock of the same character as the wall rock, occurring in the course of a vein, as of coal or ore; hence, to take horse (said of a vein) is to divide into branches for a distance.
horse: barren rock dividing a vein into branches
In mining, a horse is a body of wall rock or barren material that intrudes into an ore vein or coal seam, splitting it into separate sections. Unlike a rider, which sits on top of a vein, a horse cuts through it vertically or at an angle, forcing miners to work around it or extract ore from discontinuous segments on either side. The term appears most often in hard-rock mining, particularly in metalliferous deposits, and in coal mining where it presents a common obstacle.
A horse can be composed of any wall rock material: granite, slate, sandstone, or clay, depending on the geological setting. Its width ranges from inches to several feet, and it may persist for only a few metres along the vein's strike or extend much deeper. Horses occur because of faulting, fold deformation, or simple irregularities in the host rock as the mineralizing fluids filled the fracture. They are not ore; they yield nothing of value and must be broken and removed along with the surrounding rock.
Recognition and working around horses
A miner recognizes a horse by its sudden change in rock type and its poverty in ore. The vein will appear to vanish, replaced by country rock, then resume on the far side. This discontinuity demands careful survey work: drillers probe ahead to map the horse's extent before committing labor. In narrow, hand-worked seams, a horse forces a detour or necessitates abandonment of that section. In mechanized mines, horses are blasted and loaded as waste, adding nonproductive cost.
The term take horse describes the moment when a vein fractionalizes because of a horse. Miners say the vein has taken horse when they suddenly encounter the intrusion. The word horse itself likely derives from its bulky, unyielding character, blocking the vein much as a horse blocks a path, though etymological certainty is elusive in mining slang.
Horses are distinct from rolls, which are local undulations in a vein that do not sever it, and from feeders, which are subsidiary veins that join the main vein. A horse is fundamentally a barrier of waste rock, a geological accident that every mine must budget for and tolerate.