A circular letter arrived to-day from Aghadez, addressed to all the Tuaricks, written by Mustapha Bey of Mourzuk, recommending them to render us all necessary protection. It is dated back two months. Probably this letter was written on account of the unfavourable intelligence which reached Mourzuk respecting us. To-morrow, please God, we start for Soudan. _12th._--Thank God! we left our encampment of Chintagawna this morning. And oh, most gracious God! give us a prosperous journey, and may we be useful to ourselves and our fellow-creatures. We started about eleven o'clock, and went on about three hours and a-half. The day was very cool; the thermometer in the morning, at sunrise, being only three degrees above the freezing-point. We expect to see the water freeze on the high plains through which we are about to pass, before arriving at Damerghou. Our encampment is a pleasant wady, under a conical-formed rock of considerable elevation, perhaps 1500 feet. We are also in a high situation, some 1000 or more feet above the level of the sea. There is near this rock a lower one of an oblong form, its sides fluted with pillars; these columnar masses are basalt. Dr. Overweg examined the rocks, and found the outer crust a new species of rock, a sort of trachite or brachite; and the interior a sort of basalt, or volcanic substance. The large rock is also of the same formation. Dr. Barth ascended the large rock. I am now told that I made a great mistake about the wording of the circular letter of Mustapha Bey. This letter begins by thanking the Tuaricks of Aheer for exterminating the Walad Suleiman! It then hints broadly at the necessity for the Turks in Mourzuk and the Tuaricks of Aheer being friends; and to maintain this friendship one important condition is required--that they, the Tuaricks of Aheer, shall protect all the merchants or other travellers passing through their country, and coming from Mourzuk. In the event of their committing a bad action, the Bey says he may be compelled to make reprisals; so it is quite clear the letter is written entirely on our account, and perhaps is a preliminary measure to making reprisals. _Nous verrons._ This letter is only addressed to the people of Aheer. If water be the sustaining and even the generative force of vegetation in the desert, it is also the destruction of trees and herbage; for along the line of the current of the wady are seen immense numbers of dead and overthrown trees, torn from their roots by the force of the water in the rainy season. En-Noor paid me a visit this afternoon, and took a nap in my tent.