The 30 km2 Matchinskii intrusion is in contact with Silurian psammites and comprises three phases. These are (1) aegirine-augite and biotite-nepheline syenites, (2) peralkaline pyroxenite and quartz syenites and (3) leucocratic granites. The nepheline syenites, which occupy the western part of the intrusion, are represented by huge xenoliths within the alkaline syenites. The aegirine-augite syenites are concentrated in the central part of the intrusion and grade into quartz syenites towards the margins, the contacts with biotite syenites indicating that there was remelting of the latter. The alkaline syenites vary from mesocratic to leucocratic varieties. There are areas of quartz-tourmaline rock within the quartz syenites, in which the tourmaline may form distinct layers, while there has been extensive albitisation and carbonatisation of the alkaline syenites. The leucocratic granites form veins in the syenites. The intrusion is cut by dykes of biotite and nepheline syenite and granite. The country rocks are intensively altered with the production of alkali amphibole, pyroxene and tourmaline. Rocks referred to as carbonatites have been produced at the eastern end of the intrusion as a result of carbonatisation.