EE WAS INSTALLING ROOF BOLTS IN THE SECOND CUT OF 8 RIGHT CROSSCUT APPROX 35' INBY SPAD 741. THE EE WAS ATTEMPTING TO INSTALL STRAPS ACROSS A VISIBLE SLIP IN THE MINE ROOF AND HAD SWUNG HIS HE AD OUT TO INSTALL RIB BOLT. WHEN EE STARTED DRILLING THE HOLE THE BIT BROKE LOOSE A PIECE OF ROCK APPROX 2' X 1.5' X 2.5"" THICK STRIKING IM ON THE LEFT LEG JUST ABOVE THE KNEE.