BOLTING A PIECE OF ANGLE IRON ON THE DISCHARGE CHUTE OF A SCREENING PLANT. THE INJURED EMPLOYEEWAS KNEELING ON TOP A PIECE OF PLYWOOD WHILE ANOTHER EMPLOYEE WAS HAMMERING ON THE EQUIPMENT BE LOW. APPARENTLY, VIBRATIONS FROM THE POUNDING DISLODGED A ROCK, CAUSING IT TO FALL FROM THE SCREEN AND STRIKE THE EMPLOYEE ON THE CHEST. THE EMPLOYEE LOST BALANCE AND FELL OFF EQUIPMENT.