The linear negative magnetic anomaly which stretches continuously for 110 km across the North Minch, extending slightly west of north from Loch Ewe, is interpreted in terms of an unusually wide dyke which does not reach the surface. Fourteen profiles taken from the aeromagnetic map have been interpreted using non-linear optimization, giving an excellent fit to a dyke having an average and fairly constant width of c. 1.1km. The depth to the upper surface of the interpreted dyke varies between 0.3 km and over 2 km. It is reversely magnetized in a direction consistent with a Tertiary or possibly earlier origin. The average magnetization is estimated to be 0.51 Am−1. A similar magnetic feature on the shelf to the north-west of the Butt of Lewis is also interpreted in terms of a reversely magnetized dyke, having a poorly-resolved width of about 0.8 km, a depth of about 0.7 km and a magnetization of 1.6Am−1. On seismic evidence, the Minch dyke appears to be post-Jurassic in age, and an early Tertiary age seems most likely for both dykes. It is suggested that they may be related either to the postulated large igneous pile of early Tertiary age which forms the Wyville-Thomson Ridge, or to the Skye centre. They appear to represent early Tertiary extension of the crust which is taken up by more numerous narrow dykes further south.