The compositions of Palaeocene basic magmas in Skye and Mull, British Tertiary Igneous Province (BTIP), shed light on the general problem of the relative importance of lithospheric and asthenospheric mantle as sources of continental magmatism during extension. After the effects of crustal contamination in some magmas have been detected, and the samples showing it eliminated from consideration, the remainder show a significant diversity of trace-element compositions. Basic lavas from Skye and Mull which lack crustal contamination have contents of compatible and moderately-compatible elements (e.g., Ni, Cr, Y, Yb) and also of moderately-incompatible elements (e.g., Ti, P, Zr, Hf, Sr, MREE) which are within the range shown by the tholeiitic and mildly-alkalic ocean-island basalts (OIB), as from Iceland. In contrast, their contents of strongly- and very-strongly-incompatible elements (e.g., K, Rb, Ba, U, Th, Nb, Ta, LREE) are within the range shown by normal mid-ocean-ridge basalts. These BTIP magmas are interpreted as second-stage melts from mantle which had already lost a very small melt fraction. The latter is thought to be represented by the strongly-alkalic Permo-Carboniferous dyke swarms associated with rifting at that time in NW Scotland. Residual uppermost asthenosphere from the late Palaeozoic magma genesis event is postulated to have become attached to the thickening lithosphere beneath NW Scotland and thus to have remained coupled to it throughout the Mesozoic, until reactivated by Palaeocene stretching. Occasional hawaiites in the top third of the Skye lava pile remnant, and also basalt dykes of the swarm cutting the granites of the central complex (the last significant magmatic event at this centre) have the elemental characteristics of typical North Atlantic OIB - notably La Ta =10–12 . These compositions are thought to demonstrate input from asthenospheric-source basic magmas throughout the lithospheric-source-dominated Skye and Mull magmatism. It is suggested that all the basic magmas in this province are ultimately derived from asthenospheric sources, but that the larger magma batches became contaminated during uprise by melting and assimilating the most-fusible parts of overlying lithospheric mantle (followed by lower crust in many cases). Smaller magma batches traverse the overlying lithosphere with little or no contamination. Perhaps the difference between the two lies in their flow behaviour - laminar or turbulent - during uprise.