The almost circular intrusion of Tumbine has a diameter of 8 km and is expressed as the mountain of Serra Tumbine, which rises to 1542 m above a peneplain. The core of the intrusion consists of nepheline syenites which are surrounded by a ring of syenite. A sample of syenite described by Coelho (1959a) consists of perthite, biotite, aegirine-augite, with more sodic rims, riebeckite, scapolite and titanite. A second specimen has a gneissose fabric and consists of perthite, aegirine-augite, hornblende, biotite and a little riebeckite and apatite. Analyses of these two rocks indicate 7% and 14% normative nepheline, but quartz has been found in syenites close to the margins. A carbonatite is shown immediately southwest of Tumbine on Fig. 4.11.1 of Cilek (1989), but this may be an error.