Interpretation of two new gravity profiles across the Ben Stack line and of the I.G.S. aeromagnetic map indicates that the Lewisian biotite-gneisses occurring to the north of this line are underlain at a depth of about 3 km or less by rocks possessing similar density and magnetic properties to the pyroxene-granulites which crop out south of the line. The Ben Stack line forms the fundamental division between these two metamorphic assemblages of the Lewisian and its plane dips steeply towards the south. The gravity profiles also indicate that both the biotite-gneisses and pyroxene-granulites in the vicinity of the Ben Stack line are anomalously low in density. This is attributed to the penetration of both formations by granitic and pegmatitic intrusions of low density. The evidence suggests that the granitic material concentrated in the vicinity of the Ben Stack line occupies too great a volume to be derived solely from the host biotite-gneisses in the immediate vicinity and may therefore possibly be of more extensive origin.