EMPLOYEE RECALLS HE WAS BURSTING ON ROCK AND REMEMBERS PULLING HAMMER BACK; HAMMER HIT THE ROCK AND SOMETHING HIT HIM IN THE HEAD (INSIDE CAB OF ROCK BREAKER). THE NEXT THING HE REMEMBERED HE WAS AT DOCTOR'S OFFICE. (OTHER EMPLOYEES SAID THEY DIDN'T SEE ACCIDENT.) HE WALKED TO SHOP SEEMING TO BE IN INCOHERENT STATE OF MIND. MGR CHECKED EQUIPMENT, FOUND 3"" ROCK 1"" THICK IN CAB.