In the Mahafaly region there is an extensive dolerite dyke swarm, apparently associated with which are 10 small plugs (Boulanger, 1957a, Fig. 1) all of which consist of ankaratrite (Boulanger, 1957b); four of these are depicted on the 1:500,000 geological map (Besairie, 1970b). The plugs are subcircular, have positive relief and vary between 100 and 500 m in diameter. The ankaratrite (nephelinite) comprises olivine, augite and a little nepheline, an analysis of one being given by Lacroix (1923). Rocks referred to as limburgitic ankaratrite are also described by Lacroix (1923) and Boulanger (1957a) in which the olivine, pyroxene and nepheline are set in glass or analcime, probably the former. Rocks described as basanites of the “Southwestern dyke swarm”, which contain modal nepheline, have been analysed by Dostal et al. (1992). These rocks may derive from the Mahafaly area.