K-Ar ages on basaltic rock within the Malagarasian of Burundi and Bukoban of Tanzania are reported. Amygdaloidal lavas at the base of the Uha (Tanzania) and Mosso (Burundi) Groups are in stratigraphical continuity and give ages of 805 m.y. In Burundi, gabbros which intrude the underlying Musindozi Group (correlated with the Kigonero Flags of Tanzania) give ages of at least 890 m.y. The Kavumwe Group, which is continuous with the Bukoba Sandstone of Tanzania, is cut by quartz gabbros which are at least 1000 m.y. old. The correlation of the Kavumwe Group with the Nkoma Group which in Burundi is younger than the Musindozi Group is unsound, and this is confirmed by palaeomagnetic work which shows poles for the Bukoba Sandstone (= Kavumwe Gp), the Kigonero Flags (= Musindozi Gp), and the Gagwe Lavas and Manyovu Red Beds (= Mosso Gp) in that order on a polar wandering curve.

The Nkoma Gp is thus an independent formation situated between the Musindozi Group and Mosso Group in the time gap 890 to 805 m.y.

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