Summary The distribution of trace-elements between the Lewisian basement and the Caledonian cover-formations can be assessed from regional geochemical maps based on stream-sediment sampling. The Lewisian basement is characterised by depletion in heat-producing elements, U, K, Rb, and in Pb, Be and by high values of Sr, Mg and first row transition elements. The range of Zr, Ba, B and Y levels are comparable with those over the Moine. Regional variations relate to crustal level and to the presence of zones of high deformation. Anomalies for Mo, Zr and U are associated with granite and pegmatites in some of these zones. Within the Moine nappe the zone of tectonically emplaced Lewisian slices divides an area where the bulk geochemistry is consistent with that of continental arkoses—the Morar Division—from the Glenfinnan and Loch Eil divisions which have geochemical characteristics closer to those of greywacke facies. No other geochemical discontinuity has been recognised in the Moine but the boundary with the Dalradian is marked by an increase in boron. Clear misfits of element distribution across the Great Glen fault are apparent but the data so far available could be interpreted in terms of either sinistral or dextral displacement.