EMPLOYEE AND ANOTHER ROOF BOLTER OPERATOR WERE SCALING ROOF NEAR THE CORNER OF #1 ENTRY FROM THE #2-1 CROSSCUT OF D-6 PANEL; WHILE HE USED A 6' ROOF BOLT TO SCALE A LOOSE ROCK HE HAD OBSERVED, ANOTHER ROCK NOT OBSERVED BY THEM AND NOT BEING SCALED, FELL ON HIS OUTBY SIDE AND STRUCK HIM IN THE RIGHT LOWER LEG; A TRIANGULAR SHAPED ROCK, MEASURING APPROX. 0-3' W X 5' L X 0-12"" THICK.