loom's gave in; two were killed by the Arabs, and two were left to the chance of coming up before morning. Tremendously dreary are these marches; as far as the eye can reach, billows of sand bound the prospect. On seeing the solitary foot-passenger of the kafila, with his water-flask in his hand and bag of zumeeta on his head, sink at a distance beneath the slope of one of these, as he plods his way alone, hoping to gain a few paces in his long day's work by not following the track of the camels, one trembles for his safety: the obstacle passed which concealed him from the view, the eye is strained towards the spot, in order to be assured that he has not been buried quick in the treacherous overwhelming sand." On the 20th passed hills named Geisgal, of dark sandstone, and a table-shaped hill in the wadey Dibla, of sandstone and slate-clay. Here some fulgurites, or lightning-tubes, were observed in the sand. A number of semi-vitrified small stones were found on the sands, which the people collected to use as bullets. The journey still across sandy deserts to an extensive wadey called Aghadem, which they reached on the 23d. Here are several wells of excellent water, and hills of sandstone. From thence crossed the sand desert of Tintuma. On the 27th, "we," says Denham, "appeared gradually approaching something like vegetation. We had rising sands and clumps of fine grass the whole way, and the country was not unlike some of our heaths in England." Towards evening the trees increased in number; and when the travellers halted, the animals found abundance of food. The spot where they halted is called Geogo Balwy. They continued their route

across sands and through valleys, bounded by low sandstone hills, and by some salt lakes. As they approached the great fresh-water lake Tchad, the country improved much in appearance, owing to the increase of soil, and consequently of vegetation. On the 4th February they came in sight of this great lake. On February 5th reached Lari, on the shore of the Great Lake. On the 6th the freed slaves, natives of Kanem, left them for their homes, three days' journey to the eastward. One poor deaf and dumb woman, whom the rapacity of Mukni, the former Sultan of Fezzan, who spared neither age, sex, nor infirmity, had induced him to march to Tripoli, had shed torrents of tears ever since she had been made acquainted, by signs, that she was to go to Bornou. She had left two children behind her, and the third, which was in her arms when she was taken by the Arabs, had been torn from her breast after the first ten days of her journey across the Desert, in order that she might keep up with the camels. Her expressive motions, says Denham, in describing the manner in which the child was forced from her and thrown on the sand, where it was left to perish, while whips were applied to her, lame and worn out as she was, to quicken her tottering steps, were intensely affecting. After travelling through a wooded and beautiful country, they, on February 17th, reached Kouka. This was to the travellers an important day, as they were now about to become acquainted with a people who had never seen, or scarcely heard of, a European.

In a journey which was undertaken to Mandara, the whole country to Affagay was found to be alluvial.

Denham crossed part of a great range of mountains, named the Mandara Hills, at the most southern limit

of this journey. He says, "On all sides the apparently interminable chain of hills closed upon our view in rugged magnificence and gigantic grandeur, though not to be compared with the Higher Alps, the Apennines, or even the Sierra Morena, in magnitude; yet by none of these were they surpassed in picturesque effect." This range of mountains was found to contain granite, mica-slate, hornblende rock, and ores of iron. There were observed on the southward lower ranges of newer formation, consisting of conglomerated rocks abounding in fossil oyster-shells.

On what Formation does the Sand of the Desert rest? It is a question with geologists, on what formation or formations does the sand of the Desert rest? We have not data sufficient for a very satisfactory answer to this question. Judging, however, from the details of travellers, we would infer that the predominating formations are of the secondary class of rocks. The secondary formations met with are red and variegated sandstone, with gypsum and salt, and white and gray sandstone sometimes disposed in fantastic forms. The salt in some places is seen in thick beds, along with the red or variegated sandstones. Limestones of various descriptions, that appear to belong to the Jura limestone formation, are met with. Besides those already enumerated, there occur other limestones, clays, and gypsums, belonging to the tertiary class, from which salt springs issue. But not only these softer rocks appeared rising through the sands of the Desert; also harder rocks, as greenstone, amygdaloid, and granite, in some places project, although rarely in isolated rocks, ridges, and

cliffs. From these details it appears that the general basis of the Desert consists of secondary rocks, principally sandstone and limestone.

Description of a Trona or Natron Lake.-Natron or trona, as already mentioned, is found in various parts of the Desert, but principally in its eastern half. Dr Oudney describes, in the following terms, in a letter to us, afterwards printed in Denham's Travels, the wadey Trona he passed through in his journey from Tripoli to Mourzouk :-" Monday, July 8.We entered the wadey Trona early this morning, on the north-east side. Near where we entered there are a cluster of date palms, and a small lake, from which impure trona is obtained. On the western side the trona lake is surrounded with date-trees, and its marshy borders are covered on almost all sides by grass and a tall juncus. It is about half a mile long, and nearly 200 yards wide. At present it is of inconsiderable depth, from the evaporation of the water; for many places are dry now, which are covered in the winter and spring. The trona crystallizes at the bottom of the lake when the water is sufficiently saturated; for when the water is in large quantities it eats the trona, as the people say. The cakes vary in thickness from a fine film to several inches, (two or three). The thickest at present is not more than three-fourths of an inch; but in the winter, when the water begins to increase, it is of the thickness I have mentioned. The surface next the ground is not unequal from crystallization, but rough to the feel from numerous small rounded asperities. That next the water is generally found studded with numerous small, beautiful cubical crystals of muriate of soda; the line of junction is

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always distinct, and the one is easily removed from the other. When not covered with muriate of soda, the upper surface shows a congeries of small tabular pieces joined in every direction. When the mass is broken, there is a fine display of reticular crystals, often finely radiated. The surface of the water is covered in many places with large thin sheets of salt, giving the whole the appearance of a lake partially frozen over; film after film forms, till the whole becomes of great thickness. Thus may be observed, on the same space, trona and cubical crystals of muriate of soda; and, on the surface of the water, films accumulating till the whole amounts to a considerable thickness. The soil of the lake is dark-brown muddy sand, approaching to black, of a viscid consistence and slimy feel; and, on the lately uncovered surface of the banks, a black substance, something like mineral tar, is seen oozing out. The water begins to increase in winter, and is at its height in the spring. In the beginning of the winter the trona is thickest and best; but in the spring it disappears entirely. The size of the lake has diminished considerably within the last nine years, and, if care be not taken, the diminution will be still more considerable; for plants are making rapid encroachments, and very shallow banks are observable in many places. On making inquiry, I found the quantity of trona has not sensibly diminished for the last ten years. Perhaps it may appear so, from there always being sufficient to answer every demand. The quantity annually carried away amounts to between 400 and 500 camel-loads, each amounting to about 4 cwt.,-a large quantity, when the size of the lake is taken into account. It