The caravans that go to Bilma for salt secure it chiefly by barter, trading food and clothing to the value of their purchases. To gauge its actual value in coin, one block or cone of salt, weighing about 35 lbs., is worth two pennies in Bilma; but, when carried away south to Hausaland,[14] it is resold, or rebartered, at an entirely different value. At Tessawa it realises as much as eight shillings, or the equivalent, and at Kano ten shillings. In considering values, however, the long period spent on the journey to and from Bilma, and the loss of camels through hunger and fatigue, should be reckoned in favour of the man who brings the salt to the markets of the south, for on that account, when all is said and done, his profits at best are but little; which is all that the best type of native expects or asks. Tigguida n’Tisem is very different from Bilma, though both are renowned salt centres, and both of a character that would have assuredly made them central figures in the history of the Sahara, had the races who have come and gone through the dark ages of Africa’s existence kept comprehensive records of their country. This salt centre is not so remote as Bilma, and is easier of access from Hausaland. It lies west of Agades, and north of In-Gall, in _black desert_ beyond the mountains of Aïr. Its actual position happens to be in a region wherein tend the main lines of drainage of the rare storm-rains of western Aïr; drainage that, at the present time, seeps eventually into the desert, but that, doubtless, once ran much further on its course, which heads, even to-day, in the direction of the Niger Basin. At Tigguida n’Tisem this watercourse, remarkable because of its size, takes the form of immense flats of clayey soil, resembling the sediment of an estuary, and the salt, which is the mainstay of the town, is located in a low hill in the very centre of this strange arid bottom. Indeed, on account of its position in the watercourse, when rains do happen to occur, which is, perhaps, once a year, or once in three years, according to chance, Tigguida n’Tisem is entirely surrounded by water, and at such times the population are in the habit of trekking south to take refuge in In-Gall. But for the most part the hot sun-smitten land lies ever barren and petrified, while the wind-swept, dust-covered, diminutive town crouches, like the dens of fearful creatures, in a lost land of featureless flatness and terrible desolation. Why anyone should live there at all is beyond comprehension, until one halts at the significant word, _Salt!_ which constitutes the main occupation at present, though early geographers believed the settlement was concerned with copper. Tigguida n’Tisem is very remarkable for two reasons: the rare race of people who occupy it, and its extremely picturesque salt-pans.