The intrusive form of some basalt dykes showing flow lineation

Summary

The dilational intrusion is described of some basalt dykes of Tertiary age into the Easdale Slates at Easdale, Argyllshire. The intrusive form of the dykes results from the walls of irregular fractures, guided by pre-existing planes of weakness, moving apart under magma pressure. The propagation of the fractures is discussed in relation to the external stress field controlling the intrusion of the dykes and the direction of opening of the fractures considered in relation to the magma pressure and the external stress field. The flow lineations developed at the margins of the dykes are discussed in terms of their mode of origin. Although the attitude of these lineations varies with the intrusive form of the dykes, the overall direction of flow of the magma was upwards to the north of northwest.