Morrumbala is a multiple intrusion 15 km from north to south and up to 5 km wide. The southern half consists of peralkaline syenites, quartz syenites and a small amount of feldspathoidal syenite and the northern part of quartz syenite and younger peralkaline granites. There are numerous dykes within and in the vicinity of the intrusion. The granites comprise perthite, albite, quartz, biotite, riebeckite and rare aegirine-augite. The syenites vary from quartz-free to quartz-bearing rocks of perthite, biotite, arfvedsonite and aegirine-augite/aegirine. A small mass of nepheline syenite near the southern margin of the intrusion contains perthite, nepheline, abundant arfvedsonite, rather less aegirine, a little biotite, and accessory zircon and an opaque phase (Araújo et al., 1973). Coelho (1959a) also describes an aegirine-riebeckite-sodalite syenite containing a member of the eucolite-eudialyte group. Finer grained variants of all these rocks occur as dykes. Numerous rock analyses are given by Araújo et al. (1973) who also show a panoramic view of the intrusion; there are photomicrographs in Coelho (1959a).