Well-developed layering is expressed in size-sorting and orientation of elongate particles in four diatremes near Sydney, New South Wales. The layering was produced by flow of a very viscous aggregate during emplacement of intrusive breccia rather than by air-sorting at an open vent which does not satisfactorily explain observed field relations. The size of particles in any particular layer depends on the distance between flow planes and only subordinately on relative velocities of flow in each layer. Some banded structures in non-brecciated igneous intrusions are also briefly considered; the conclusion is that a similar process can cause mineralogical banding which may be further modified by dilatant effects.