EMPLOYEE STATED THAT HE WAS OPERATING CONTINUOS MINER (REMOTE CONTROL) WHEN A PIECE OF ROCK (24 INCHES BY 18 INCHES FROM 0-3 INCEHS THICK) FELL FROM THE TOP. (A DISTANCE OF 3.5 FT) AND STRUCK THE TOP OF THE HARD HAT. PIECE OF ROCK FELL FROMBETWEEN TWO OUTSIDE BOLTS.