EMPLOYEE WAS INSTALLING ROOF BOLTS ON UNIT #1 AT ENTRY #3 IN THE FACE. SOME ROCKS HAD DROPPED OUT IN THE FACE AREA. AS INJURED WAS INSTALLING A ROOF BOLT A ROCK FELL OUT BETWEEN THE RIB AND RI B BOLT. HE WAS STRUCK ON THE NECK, BACK, AND RIGHT LEG. CONTUSION TYPE INJURY. ROCK MEASURED 4' X 18"" X 2"".