The mica-apatite occurrences studied lie in North Burgess Township, Lanark County, Ontario. The rocks surrounding the deposits are dominantly crystalline paragneiss and limestone of the Grenville series. Their lithological variations stem from differences in the original composition of the sediments and the varying degree of metamorphism and injection by pegmatitic material which the rocks have undergone. Evidence is presented for the origin of the pyroxenite in the mica-apatite deposits through contact metamorphism of crystalline limestone by pegmatitic solutions. The close association of pyroxenite with mica and apatite in all the deposits examined suggests that pegmatitic solutions were responsible for the deposition of these minerals. Several lines of evidence support this view. Field and experimental information point to the sedimentary origin of the pink carbonate frequently found in the mica-apatite pits.