The geological structure of the Sahara is really very simple. The true Sahara is just the opposite of a young folded range. Throughout the remainder of the Sahara there are immense areas where the Carboniferous limestones, Devonian sandstones, and even Silurian sandstones appear in beds that are almost horizontal. There are immense stretches comprising perhaps half the Sahara, where this substratum or foundation disappears under an overlay of more rocks. The most outstanding of the mountainous massifs of the Sahara, the Tibesti, the Air and the Ahaggar, are all volcanic. Anything, indeed, that rises sharply the general level of the desert platform is more than likely to be of volcanic nature. The series of volcanoes aligns itself roughly but plainly in an east-west direction across the Central Sahara between the Tibesti and In Ziza, passing through the Aïr and the Ahaggar.