In the present communication I propose to describe the petrographical characters of some hornblende-bearing rocks which occur as intrusive sheets and bosses in the limestones and quartzites of the Assynt district. These rocks have been referred to under various names by the different writers on the geology of the district. Prof. Nicol speaks of them as greenstone and trap, and Sir R. Murchison, as syenitic greenstone. Mr. Hudleston indicates their character more definitely by speaking of them as a “kind of diorite.” Prof. Bonney, in an appendix to Dr. Callaway's paper “On the Newer Gneissic Eocks of the Northern Highlands,” describes the microscopic structure of a specimen from the Traligill Burn near Inchnadampf, and designates the rock a hornblendic porphyrite. Dr. Hoddle gives some valuable information as to the distribution of the rocks, and calls attention to the perfection of form exhibited by the hornblende in certain varieties. He gives a figure of the hornblende which shows the forms (110), (010) and (011). With regard to the relations of the rocks to the associated strata he speaks somewhat doubtfully, but evidently inclines to the view that they are intrusive. That this is the case seems proved by the absence of vesicular structure and associated tuffs, and by the fact that although they keep as a rule parallel with the bedding, cases occasionally occur in which they can be seen to move from one horizon to another.