As a preliminary to an account of the marscoite suite of the Western Redhills Tertiary igneous centre of Skye, a description is given here of one unit, the ferrodiorite. While accepting Harker's hypothesis of the hybrid origin of marscote (1904, pp. 175–196) we consider that the ferrodiorite, with which marscoite on Marsco is associated, is not itself a hybrid, but one of the parents from which the hybrids were formed. Among Thulean igneous rocks the ferrodiorite has mineralogical and chemical affinities with Hebridean mugearites, on the one hand, and with the ferrogabbros of the Skaergaard intrusion on the other, but is probably significantly different from both.