The early major products of Tertiary volcanicity in both Skye and Mull are transitional basic lavas, similar in their major-element chemistry to world-wide alkali basalt series. In contrast, their contents of incompatible trace elements bear more resemblance to those of olivine tholeites. The Mull basalts have similar ranges of silica saturation, Mg/(Mg+Fe), Y and Yb, but lower overall abundance ranges of strongly incompatible elements than the Skye basalts. The variation of incompatible elements in the Mull and Skye lavas is consistent with a model of a mantle source from which a small amount of melt (no more than 1%?) had been extracted, with the pre-Tertiary upper-mantle fusion beneath Mull slightly greater than beneath Skye. Chemical and tectonic considerations suggest that this mantle was neither residual from the formation of the Archaean Lewisian complex, nor emplaced as a result of tension associated with the Cainozoic rifting of the North Atlantic. Data on major and trace elements for a mafic alkalic dyke of the Permian swarms that pass through western Scotland show that these have the requisite geochemical characteristics to have caused this depletion. Such dykes are more abundant in the region of Mull than Skye.