The chemical compositions of eighty granitic rocks from the Lewisian of the North-West Highlands of Scotland, particularly from the Gairloch district, are set out in relation to geological occurrence and their normative proportions of albite, orthoclase, quartz, and anorthite compared with experimental data relating to the systems NaAlSi₃O₈-KAlSi₃O₈-SiO₂-H₂O and KAlSi₃O₈-NaAlSi₂O₈-CaAl₂Si₂O₈-SiO₂. The field of composition for these Lewisian rocks moves from Ab-Q-rich for autochthonous granites to Ab = Or = Q (approximately) for parautochthonous granites to Or-rich for intrusive granites. This trend is related to the varying roles of mineral solubility under stress, selective melting, and potassium metasomatism.