Abstract
The group of large, positive, magnetic anomalies in south-central England is shown to originate from two sources at different depths. The shallow source originates from Silesian basic igneous rocks. The anomaly of the deeper source has been isolated by upward continuation and inverted via pseudofield transformation. The magnetic model is interpreted as a large body of basaltic rocks intruding the middle crust. It is proposed that the body was emplaced by buoyant ascent and may be the cause of the stability of the London Platform during Mesozoic times. It is speculated that the basalt was intruded in a back-arc environment during the period preceding the Variscan orogeny.