EMPLOYEE WAS ROOF BOLTING A CROSSCUT BETWEEN 3-2AT THE TIME OF THIS INCIDENT. A LARGE ROCK HAD FALLEN IN FRONT OF THE ATRS. HE WAS USING A PRY BAR, STANDING UNDER SUPPORTED TOP, TO PRY DOWN A LOOSE ROCK THAT MEASURED 40"" X 34"" X 3"" WHEN THEROCK FELL AND HIT THE PREVIOUS ROCK, SLID DOWN &STRUCK HIM ON THE OUTSIDE OF HIS LEFT KNEE.