EE WAS REMOVING THE BOLTS FROM THE MAIN PANEL ON#18 COAL HAULER AT THE TIME OF THIS INCIDENT. A ROCK MEASURING APPROX. 3'X 18'X 4' FELL FROM THEROOF STRIKING EE ON THE TOP OF HIS HEAD AND THEN MIDDLE OF HIS BACK. THIS INCIDENT CAUSED A POSSIBLE COMPRESSION FRACTURE OF A VETEBRAE IN THE THORACIC REGION OF HIS BACK.