The results of reconnaissance melting experiments on Tertiary granophyres and Torridonian arkose at 1000 kg/cm² water-vapour pressure show that the melting behaviour of these rocks can be viewed in relation to the synthetic system NaAlSi₃O₈-KAlSi₃O₈-SiO₂-H₂O. By analogy with this system, P-T curves are constructed for Skye granophyres and, viewed in relation to the stability of tridymite, the depth and temperature of emplacement are estimated. Further estimates of the likely depth of magma generation, and the behaviour of the country rocks at that level, lead to the conclusion that most of the Tertiary granitic rocks were produced through partial melting of Lewisian basement rocks, the heat being provided by the basic magmas available during this volcanic episode. Subsidiary amounts of granitic magma were similarly derived from higher levels, within the Torridonian arkose group, and evidence is provided for the depth and temperature at which these, together with tridymite-bearing metamorphosed arkose in Rhum, were produced.