Overlying Precambrian schists, granites and volcanic rocks in the vicinity of Zaker are phonolitic tuffs and lavas which for the most part form small (<1 km diameter) areas with one larger, irregular occurrence extending east-west for 11 km. The thickness is generally less than 100 m. There are also about a dozen volcanic necks of phonolitic breccia and tuff and to the north of Sarho occur dykes of what are described as ankaratrites by de Sitter et al. (1952). Three types of phonolite are distinguished: normal, mela- and hauyne-bearing. The normal type contain phenocrysts of alkali feldspar, nepheline, sometimes altered to zeolite, and aegirine in a matrix of sanidine, nepheline, aegirine and an opaque phase. Spherical structures in these rocks consist of oligoclase and aegirine. The hauyne-bearing phonolites are similar to the normal type with the addition of hauyne phenocrysts and the presence of secondary calcite and accessory apatite and titanite. The mela-phonolites have partially altered phenocrysts of olivine, aegirine-augite, biotite and hauyne with dark rims, in a matrix of sanidine, augite, aegirine and nepheline. Phenocrysts in the ankaratrites are olivine, titaniferous augite with rims of aegirine, biotite and brown amphibole which are set in a matrix of pyroxene, nepheline and a little sanidine. Rock analyses are given by de Sitter et al. (1952).