EMPLOYEE HAD FINISHED BOLTING AND WAS ROCK DUSTING WHEN THIS INCIDENT OCCURRED. ANOTHER EMPLOYEEWAS SPOTTING AN EXTRA BOLT, INJURED HAD WALKED TO HIS SIDE OF THE BOLTER AND WAS THROWING ROCK D UST ON THE ROOF AND RIBS. A ROCK MEASURING 18"" BY 8"" BY 1"" FELL FROM THE ROOF AND STRUCK HIM ON THE LEFT HAND. IT LACERATED THE RING FINGERON LEFT HAND.