The xenolithic Tertiary ultrabasic dykes of south-west Skye are composed principally of forsteritic olivine, plagioclase, and clinopyroxene with accessory chrome spinel and magnetite. The plagioclase: pyroxene ratios are relatively constant within each dyke but the absolute amounts of these minerals are dependent on the olivine content which varies, often considerably, across the dykes. Five main types of olivine distribution are recorded from the dykes and in all five there is a concentration of olivine crystals towards the centre of the dyke. The average size of the olivine crystals also varies across the dykes with increases in size generally corresponding to increases in the olivine content. The petrogenetic hypothesis of composite intrusion previously proposed for these dykes by Bowen (1928) is re-examined and found to be inadequate. It is suggested that the dykes, which were intruded as magmatic suspensions of olivine crystals and rock fragments, are unlikely to have been differentiated in their present positions and the possibility that the differentiation occurred during their emplacement is examined. It is shown that the mineral distributions and crystal size variations occurring in the dykes are, on the whole, analogous to those expected to arise during laminar flow in vertical conduits, and it is concluded that flow differentiation occurred during the intrusion of the ultrabasic dykes of south-west Skye.