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 Ꮓ 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 is only removed from the lake when a demand comes. A man goes in, breaks it off in large pieces, and those on the banks remove the extraneous matter, and pack it in large square bundles. The wa ter in the valley is good, being free from saline impregnation."-Clapperton, Denham, and Oudney's Journal, p. 57. Fulgurite and Meteoric Iron found in the Desert.—In some parts of the Desert, tubes of sand, resembling those found at Drigg in Cumberland, and in different sandy districts on the continent of Europe, are met with. They are named fulgurites, or lightning-tubes, by naturalists, and are supposed to be formed by the lightning striking through the sand, and partially melting portions of it. Masses of meteoric iron also have been met with in the Desert. Golberry, in his journey through Western Africa, in the years 1805-7, mentions his having found a mass of meteoric iron in the Desert. Fragments of it were brought to Europe by Colonel O'Hara, and were analysed by Mr Howard, who found it composed of ninety-six parts of iron and four of nickel. Observations on the Sand of the Desert.-Having now noticed the rocks and some of the minerals met with in the Desert, we shall next attend to the sand of which it is principally composed. The loose alluvial matter, which forms the sand of the Desert, is principally composed of particles of white and gray quartz of various sizes, generally very small, forming the sand, properly so called, seldom so large as to form gravel and pebbles. Some are of opinion that this sand is an original deposite; others, that it is formed from previously-existing rocks through the agency of water. Moving Pillars of Sand in the Desert.-During the storms that often rage in this Desert, the sand is raised into clouds that obscure the horizon, or it is by whirlwinds raised into pillars. Bruce describes an appearance of this kind, which he witnessed in his journey through the eastern part of the Desert, in his route to Abyssinia, in the following terms:-" At one o'clock we alighted among some acacia-trees at Waadi-el-Halboub, having gone twenty-one miles. We were here at once surprised and terrified by a sight, surely one of the most magnificent in the world. In that vast expanse of Desert, from west to north-west of us, we saw a number of prodigious pillars of sand at different distances, at times moving with great velocity, at others stalking on with majestic slowness. At intervals we thought they were coming in a very few minutes to overwhelm us, and small quantities of sand did actually more than once reach us; again they would retreat, so as to be almost out of sight, their tops reaching the very clouds; then the tops often separated from the bodies, and these, once disjoined, dispersed in air, and did not appear more; sometimes they were broken in the middle, as if they were struck with large cannon-shot. At noon they began to advance with considerable swiftness upon us,—the wind being very strong at north. Eleven ranged alongside of us, about the distance of three miles; the greatest diameter of the largest appeared to me at that distance as if it would measure ten feet. They retired from us with a wind at south-east, leaving an impression on my mind to which I can |