INJURED AND SUPERVISOR WERE STANDING OUTBY THE CONTINUOUS MINER WATCHING COAL BEING LOADED INTOTHE COAL HAULER. A PIECE OF ROCK HAD FALLEN ONTOHEAD OF MINER. WHEN THE PIECE OF ROCK CAME BACK THE CONVEYOR CHAIN, IT FELL OFF AND STRUCK INJURED ABOVE THE RT. KNEE. SUPERVISOR HAD SEEN THE ROCK AND PULLED INJURED BACK.