Employee was bolting in the No. 2 heading & had installed th first row of bolts in the cut. He had installed the first bolt in the second row & was moving from the drill controls to the tram controls on the side of the bolter when a slug of rock 49"" long X 20"" wide X 0"" - 7"" thick fell from the rib line over between the roof bolts installed striking him on the lower back & hips.