Alkaline rocks are found in a number of small outcrops along the Rayfield River, but are completely surrounded by younger Miocene- Pliocene plateau olivine basalts. The outcrops appear to represent part of a pluton zoned from marginal monzonite through amphibole syenite to a central leucocratic syenite. Microcline perthite is the dominant feldspar throughout with a little interstitial sodic plagioclase in the leucosyenite. Amphibole, much altered to biotite, is the principal mafic phase, with a little pyroxene, muscovite, garnet, apatite and sphene. All the rocks have a trachytic texture. Small leucocratic syenite and nepheline syenite pegmatite dykes are numerous.