Chilallo is a large volcanic cone on the eastern shoulder of the Ethiopian rift valley. It is elliptical in plan covering 30x20 km and has a 6 km diameter summit caldera around the inner rim of which are very large, radially disposed dykes. The slopes of the volcano are described by Di Paola (1973) as consisting of thick alkaline trachytic lava flows, whereas Trua et al. (1999) refer to basaltic-trachyandesitic lava flows interbedded with strombolian scoriaceous layers. What is described by Trua et al. (1999) as the ‘Eastern Margin unit’ comprises a pile of basaltic lavas overlain by pantelleritic ignimbrites. The Chilallo and Eastern Margin rocks range from basalt through hawaiite and mugearite to benmoreite. The pantellerites have eutaxitic textures with flattened pumice and glassy shards and contain fragmental crystals of quartz and sanidine and sparse phenocrysts of oligoclase; pyroxene is sodium-rich augite. A detailed geology map, in colour, of the western part of Chilallo and the areas to the north (No. 41) and west, also gives summaries of the petrography and rock chemistry (Abebe et al., 1998), Analyses of 14 rocks, including a range of trace elements and Sr, Nd and Pb isotopic data are in Trua et al. (1999). Mohr (1966a), in the absence of thin sections, referred to "the trachyte-carbonatite volcanic centres of Chilala and Kakka" and considered that they have "essentially identical petrology with the Wachacha-Yerer suite" (No. 31). However, Mohr (pers. comm., 1995) says the identification of carbonatite was in error.