THE INJURED WAS SWITCHING OUT #72 BUS IN THE MAIN TRIP SPUR IN THE ROCK DUST HOLE ON THE BOTTOM OAT THE ELECTRIC SWITCH. HE WAS TRAMMING #72 BUSINTO THE SPUR AS HE LOOKED UP AT THE WIRE AND TH EN HE WAS STRUCK ON THE BACK OF HIS HEAD BY A FALLING ROCK. HE WAS TALKING OUTSIDE WHEN INVESTIGATED THE #72 BUS HAD ALREADY LEFT THE ROCK DUST HOLE. WE CHECKED THE AREA WE FOUND A PIECE OF R