EMPLOYEE WAS ROOF BOLTING WITH A COBRA DRILL AT #72 SHEILD F-9 LONGWALL FACE WHEN A ROCK THAT MEASURED APPROX. 1'THICK X 2 1/2'WIDE X 15'LONG FELL & DISLODGED THE ROOF JACK THAT WAS SET. AS THE EE WAS MOVING OUT OF THE WAY. THE ROOF JACK STRUCK HIM ACROSS THE LOWER BACK CAUSING HIM TO FALL HITTING HIS NECK AGAINST THE RAIL OF PANELING. EE RECEIVED 3 FRACTURED VERTEBRAE.