Employee was walking around roof bolter where a piece of rock had just been pulled on the opposite side of the machine when a piece of rock measuring 24"" x 36"" x 1.5"" fell out from between rows of bolts and struck him on top of the head. Bolts were 3'4"" from rib and 4' from row to row. Put on restricted days 5/4/11 - 5/7/11.