Basic and ultrabasic rock masses forming hills at the western end of Lleyn peninsula, Wales, have been intruded in lower Ordovician slates and tilted during Caledonian crustal movements. Their age is probably mid-Ordovician. Study of the intrusive masses near Rhiw shows that the banding previously reported to have been observed in hornblende picrite and dolerite represents at least three major units of rhythmic layering. Processes involved in development of the layering probably included gravitative accumulation of early formed crystals, winnowing by currents within the magma, and episodic introduction of new magma similar to the more prolonged processes responsible for formation of the Stillwater, Skaergaard, and Rhum layered complexes.