EMPLOYEE & A SECTION MANAGER WERE SCALING ROOF NEAR THE CORNER OF #1 ENTRY FROM THE #2-1 CROSSCUT OF D-6 PANEL. AS EMPLOYEE USED A PICK HAMMER TO SCALE A LOOSE ROCK HE HAD OBSERVED, ANOTHER R M & NOT BEING SCALED--FELL ON HIS OUTBY SIDE & STRUCK HIM ON LEFT ARM ON THE ANTERIOR SIDE AT THE ELBOW, A TRIANGULAR SHAPED ROCK MEASURING APPROX. 0-3' W X 5' L X 0-12"" THICK; CAUSED 5"" CUT.