The volcano and the crater at its foot are not the only indications of former igneous action in the district, now almost quiescent since the disappearance of the ancicnt inland waters that washed the western foot of the hills. A thermal spring, famous throughout the eastern Sahara, flows in a ravine east of the main range, some thirty miles to the south of Bardai, the chief oasis in Tibesti. This spring, known as the Yerikeh, or "Fountain," in a pre-eminent sense, is said to be so hot, and to emit such continuous jets of steam, that it cannot be approached. The reports of explosions are also incessantly heard in the midst of the vapours. Nachtigal was not permitted to visit this hot spring, which was described by the
natives as their "only wealth." They doubtless feared he might obtain by magic art the gold mines supposed to be concealed by the jins in the neighbouring rocks. The cavities in the immediate vicinity are filled with deposits of sulphur.
Althovgh the breadth of the Tibesti range cannot yet be determined, it seems certain that towards the central part it shows a development of over 60 miles transversely to its axis, from the plain of Borku to the Libyan desert. On the whole, the southern slopes are less abrupt than those on the opposite side. Here begins the broad plain which stretches away without perceptible incline in the direction of the Kufra oases. Towards the north-west the range is interrupted by wide depressions, separating from each other some groups of steep or even inaccessible rocks. One of these, to the north-west of Tarso, is composed of sandstone blocks, which assume the most varied architectural forms — Roman amphitheatres, Byzantine churches, frowning fortresses. Interspersed among these more regular structures, which have a mean elevation of about 200 feet, are the fantastic outlines of men and animals.
Farther on, along the same north-westerly prolongation of the Tibesti range, rise the mountains of Abo, 1,830 feet high, followed successively by those of Afafi,
