The object of this communication is to describe the gradual change from dolerite to hornblende-schist as it may be observed in two more or less parallel dykes which occur in the Archæan gneiss of the north-west of Scotland, near the village of Scourie in Sutherlandshire. The northern promontory of Scourie Bay is named on the Ordnance map Creag a'Mhail. Near this point two dykes of basic igneous rock may be seen. The one to the south is about 30 yards wide; it has been more affected by denudation than the surrounding gneiss, and its course is thus marked by a depression which is occupied by the sea at high tide. A short distance to the north of the actual promontory is another dyke, about 20 or 30 feet wide, of similar character, which terminates westward in a vertical cliff. This may be traced in a south-easterly direction down to a small beach where the southern dyke is again seen, and from this point the two dykes may be followed, running parallel to each other towards the south-east, for a distance of a quarter of a mile or more. The prevalent strike of the gneissic banding in this district is E.N.E. and W.S.W., with a moderate dip to N.N.W. The dykes therefore cut across the strike and cannot possibly be regarded as bands in the gneiss. On the opposite shores of Scourie Lake (Loch a' Bhaid Daraich) and close by the side of the road to Laxford Bridge (three quarters of