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.
HACKMAN, B.D., CHARSLEY, T.J., KAGASI, J., KEY, R.M., SIAMBI, W.S. and WILKINSON, A.F. 1989. Geology of the Isiolo area. Report, Mines and Geology Department, Nairobi, 103: 1-88.