It is filled with small volume tephritic phonolite and phonolite lava flows. It can be described as a mafic phonolite or a potassic tephrite. These include nephelinite, basanite and peralkaline phonolite, trachyte and comendite lavas. The volcanoes are all surmounted by rocks composed of trachyte, phonolite, pantellerite, or comendite. Rock types are made up of phonolite and ignimbrite. Late Tertiary faulting and volcanism gave rise to basalt and phonolite dikes. It is made of a type of rock called phonolite. Units within the complex include carbonatite, nepheline syenite, phonolite, and ijolite. They are composed of trachyte, trachyandesite, basalt, phonolite, basanite and to a lesser extent phonotephrite. These include nephelinite, basanite and peralkaline phonolite, trachyte, and comendite lavas. The Bass is a phonolite volcanic plug. Blairmorite is an analcite-rich variety of phonolite. Later activity produced trachyte and phonolite plugs and lava flows. Atakor has erupted basalts, phonolite and trachyte, the latter two form lava domes.