De Kock (1934) described and mapped phonolites and some trachytes in the Duruchaus area, but it is not clear from the published information whether or not this is a continuation of the alkaline igneous activity of the Auas Mountains (No. 116-00-021) a short distance to the northeast. He identified 20 occurrences, the majority of phonolite but some of trachyte, many of which form inselbergs in otherwise relatively flat country. Plugs, flows, sills and dykes are represented but all the larger bodies are sills. The phonolites contain alkali feldspar, nepheline and aegirine, both as phenocrysts and in the groundmass, and a little biotite. One rock analysis is available. The trachytes are feldspar-phyric and contain a little nepheline; other minerals are too altered to be identified.