EMPLOYEE WAS ROOF BOLTING IN A CM SECTION. THE ROOF BOLTER WAS PULLED INTO A FRESH CUT, ATRS PRESSURIZED AND EMPLOYEE HAD INSTALLED THE OUTSIDE ROOF BOLT. HE SWUNG HIS DRILL HEAD IN AND BEGAN TO DRILL THE INSIDE BOLT HOLE. A PIECE OF ROCK @ 7' X 4' X 13"" BROKE FREE, FELL FROM ROOF, GLANCED OFF CANOPY AND DRILL HEAD AND A PEICE HIT EMPLOYEE ON RT LEG BELOW KNEE.