The summit of the Sabinyo volcano marks a triple international boundary junction between Congo, Rwanda and Uganda. About half of the products of the volcano lie in Rwanda and these are described under Rwanda No. 3.
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The greater part of the Visoke volcano lies in Rwanda and a description will be found under Rwanda No.038-00-00 4.
Some 80% of the Karisimbi volcano is in Rwanda and a description will be found under Rwanda No. 038-00-005.
Rusongati is a 7 km long ridge orientated west northwest-east southeast at the western end of which nepheline syenite has been found, the same rock also occurring in a stream running southwards off the eastern end of the ridge. Outcrop is poor, the area being mantled by recent volcanic ash.
An elliptical intrusion of 8x3.5 km Numbi is emplaced in Precambrian Burundian schists and quartzites.
Kahusi comprises both intrusive and extrusive suites which occupy an area of about 9x9 km. There is, however, a considerable disparity in the ages of the suites (see below) which inevitably makes their petrogenetic relationship problematical.
This complex is only marginally alkaline. It consists of two intrusions of approximately 3.5x0.6 km elongated east-west and separated by a narrow zone of Burundian-Kibaran mica schists. The northern intrusion is of microsyenite: the southern of microgranite.
Biega, like Kahusi just to the north, consists of an intrusive suite of granites and syenites which are partly overlain by volcanic rocks. The complex is intruded into Precambrian metamorphic rocks of the Kibaran and Burundian systems.
The Bukavu volcanic field lies to the south and southwest of Lake Kivu and extends for some 80 km from north to south. The somewhat smaller Kamituga volcanic field lies further to the south-southeast.
This occurrence is listed, without details, as a carbonatite by Kun (1961), but carbon and oxygen isotope determinations by Pineau and Javoy (1969) and Denaeyer (1970) indicate that this is an occurrence of sedimentary limestones.