A large part of Central Sutherland is occupied by an injection complex in which rocks of the Moine Series are associated with much granitic and pegmatitic material. These migmatitic Moinian rocks have reached a higher grade of metamorphism than those which occur outside the injection complex. One feature characteristic of the high-grade migmatites is the presence of sillimanite in many of the pelitic and semi-pelitic rocks. Near the village of Kildonan, ten miles north-west of Helmsdale, sillimanite is not only abundant in the country rock, but occurs also in many pegmatitic and aplitic veins. The field and microscopic evidence shows that this mineral was formed as a result of metasomatic activity at a late stage in the history of the injection complex, when the general metamorphism was already on the wane. The sillimanite seems to have no direct connection with the conditions of regional metamorphism. It was formed under the influence of pegmatitic solutions. In view of the common use of this mineral as an index of the grade of regional metamorphism, it is of interest to describe the evidence on which the above conclusions are based.