EMPLOYEE WAS IN THE SECOND CHUTE OF THE DRY DECKTO CLEAN OUT MATERIAL BUILDUP; NOT SEEING HIM INTHE CHUTE, ANOTHER EMPLOYEE CLIMBED ON TOP TO CLEAN UP MATERIAL. HE TOSSED A 6"" ROCK OFF AND IT WENT THROUGH THE CHUTE GATES AND STRUCK THE FRONT OF VICTIM'S HARD HAT; AFTER HE CLIMBED OUTHE COMPLAINED OF A STIFF NECK AND NAUSEA.