EE WAS OPERATING THE LEFT SIDE OF THE ROOF BOLT MACHINE & INSTALLING THE OUTSIDE BOLT STANDING NEAR THE RIB LINE WHEN A PIECE OF ROCK MEASURING 4' X 8"" X 8"" FELL OUT FROM THE COAL RIB STRIKING HIM ON THE RIGHT LOWER LEG. THE ROCK CAME FROM APARTING LOCATED IN THE COAL SEAM ~3' FROM THE MINE FLOOR. THE COAL HEIGHT IN THE AREA WAS 7' TO 8'. EE STATED HE DID NOT DETECT THE LOOSE ROCK.