The Ali Sabieh-Aysha horst extends southwards into Ethiopia and along its northwestern and eastern margins are rhyolitic rocks which are correlated with the Mablas rhyolites of the North Tadjura area (No. 2) (Varet, 1978). The more easterly group are referred to as the Ali Adde rhyolites and these are older than the northwestern group of rhyolites. The rhyolites are peralkaline and the brief petrographic details given for the North Tadjoura area apply also to Ali Sabieh (Gadalia and Varet, 1983). Major and trace elements, including REE and Sr, Nd and Pb isotopic data, are given for rocks by Deniel et al. (1994).