Scattered over this large area are numerous small intrusions emplaced in lower Cretaceous sediments. There appear to be no detailed petrographical accounts but the general description of Farrington (1952) indicates that a broad range of rock types is involved, including some which are alkaline. The intrusions include dykes, sills and plugs up to 2-3 km diameter and at least one laccolith. Rock types apparently range from gabbroic to syenitic types with 'augite' and 'hornblende' the commonest mafic phases and less commonly biotite and aegirine. Coulon et al. (1996) refer to domes and necks of phonotephrite, tephriphonolite, phonolite and trachyte in the Katyo area but no further details are given.