EMPLOYEE WAS USING A SLEDGEHAMMER TO BEAT ON THE SIDE OF A ROCK BOX WHICH HAD PLUGGED. AS HE WAS HITTING THE ROCK BOX, HIT STRUCK HIS LEFT HAND AT ABOUT THE THIRD KNUCKLE. EMPLOYEE DID NOT REQ UEST TO SEE A DOCTOR UNTIL 03-15 BECAUSE HE FELTHIS HAND WOULD BE OK. HE SAW DOCTOR ON 031-17 AND WAS PLACED ON RESTRICTED DUTY, HOWEVER, HE DID NOT REPORT IT TO US UNTIL 03-23 BECAUSE HE WAS