Kapiri Hill is an east-west-trending ridge of Precambrian dolomite which is cut by several dykes of carbonatite. The largest dyke comprises carbonatite agglomerate at its northern end and carbonated nephelinite at the southern extremity. The carbonatite consists of fine-grained ankerite, patches of fine-grained quartz, and in places calcite, apatite, barite and possible monazite. Northwards the carbonatite becomes choked with xenoliths of fenitized gneiss and southwards the rock is again filled with fenite xenoliths and consists of very fine-grained carbonate, abundant apatite needles, quartz, possible perovskite much replaced by carbonate and occasional phenocrysts of biotite. Garson (1965b) considered this rock to be carbonated nephelinite. Other carbonatite dykes are dominantly iron-rich varieties with apatite, late quartz and calcite and accessory barite and probable synchysite. An irregular dyke 1.2 km northwest of Kapiri Hill is up to 30 m thick and nearly 500 m long and again is considered to be carbonated nephelinite. The least altered portions of this rock consist of phenocrysts of pale green pyroxene, brown amphibole, orange biotite, rare serpentinized olivine and probable perovskite. The matrix comprises altered nepheline, tiny laths of colourless pyroxene, an opaque phase, xenocrysts of turbid feldspar and vesicles of calcite and zeolite. There is a wide fenite envelope to the northern part of the dyke and a half metre wide fenite aureole elsewhere. There are dykes of monchiquite and phonolite in the Kapiri area.