EMPLOYEES' WERE BOLTING IN #1 HEADING WHEN THEY HAD FINISHED A ROW OF BOLTS AND WERE ADVANCING THE MACHINE FOR THE NEXT ROW. THEY SET THE T-BARINJURED STARTED DRILLING FOR HIS RIB BOLT WHEN A PIECE OF ROCK MEASURING 3'X 4'X 1"" TO 4""THICK FELL AND STRUCK HIM ON HEAD AND BACK. THE ROCK FELL FROM THE RIB TO WHERE HE WAS DRILLING.