In the extreme northwest of the Isiola area occur extensive phonolite flows, one of which extends some 20 kilometres southeastwards along a former river valley - the ‘Barsalinga tongue’ (Hackman et al., 1989, Fig. 13). A small area of these phonolites is found in the southwest of the Laisamis area (No. 085-00-027). They are considered to form part of the Rumuruti group of phonolites, that spread extensively over the Laikipia Plateau of the Baringo-Laikipia area (No. 085-00-032). There are at least two flows in the Barsalinga area the lower one being about 3 m thick and fine-grained. The upper flow is up to 20 m thick and is characterised by aligned alkali feldspar and nepheline phenocrysts which increase in size upwards, the former reaching a maximum length of about 5 cm and the latter 3 cm. The fine-grained phonolites contain katophorite in the groundmass while the coarse porphyritic ones have a groundmass of alkali feldspar, augite, aegirine-augite, magnetite and possible aenigmatite and katophorite.