EMPLOYEE WAS USING 988 B CAT LOADER TO CAST MATERIAL OFF #3 BENCH TO QUARRY FLOOR. AS HE WAS FILLING BUCKET, A BOULDER ROLLED BACK INTO BUCKET AND A PIECE (ABOUT HARD HAT SIZE) BROKE OFF AND S TRUCK WINDSHIELD. A PIECE OF GLASS FROM WINDSHIELD CUT EMPLOYEE'S LEFT ARMS. MUCK PILE HEIGHT WAS ABOUT 15 FT AND WORKING UP AN APPROX 10% GRADE.