An investigation has been made of the chemical and mineralogical transformations which have accompanied the weathering of six basic volcanic rocks from eastern New South Wales. Montmorillonite is initially formed from the destruction of the olivine, pyroxene, and plagioclase in the parent rock, but, in the near-surface horizons, this mineral is unstable and may form kaolinite, halloysite, or a poorly crystalline montmorillonite. The alumina content shows a steady increase up the profiles, whereas the alkalis, alkaline earths, and some of the silica have been lost in the ground-waters during the mineral transformations. Marked concentrations of iron and titanium occur at specific levels in some of the weathering profiles and explanations of these phenomena are given.