Roof bolter was being positioned to install 4' resin bolts & mats. As the machine approached to final 2 rows of bolts previously installed so as to begin bolting the new cut, a piece of rock fell from between the bolts striking the victim on the back. Rock Appx 12"" w, 3' long. ATRS & post jacks not yet deployed as the roof bolter had not yet reached it's beginning position.