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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 cannonshot. 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 the largest appeared to me at that distance as if it would measure ten feet. They retired from us with wind at south-east, leaving an impression on my mind to which I can give no name, though surely one ingredient in it was fear, with a considerable deal of wonder and astonishment. It was in vain to think of flying; the swiftest horse would be of no
use to carry us out of this danger, and the full conviction of this riveted me to the spot." A similar account of these moving pillars of sand is given by M. Adanson, who had an opportunity of observing one of them crossing the river Gambia from the Great Desert. It passed within eighteen or twenty fathoms of the stern of the vessel, and seemed to measure ten or twelve feet in circumference, and about 250 feet in height. Its heat was sensibly felt at the distance of 100 feet, and it left a strong smell, more like that given out by saltpetre than sulphur, and which remained a long time.
Sand-wind. The overpowering effects of a sudden sand-wind, when nearly at the border of the Desert, often destroy a whole kafila, already weakened by fatigue. "Indeed," says Denham, "the sand-storm we had the misfortune to encounter in crossing the Desert gave us a pretty correct idea of the dreaded effects of these hurricanes. The wind raised the fine sand, with which the extensive Desert was covered, so as to fill the atmosphere, and render the immense space before us impenetrable to the eye beyond a few yards. The sun and clouds were entirely obscured, and a suffocating and oppressive weight accompanied the flakes and masses of sand which, I had almost said, we had to penetrate at every step. At times we completely lost sight of the camels, though only a few yards before us. The horses hung their tongues out of their mouths, and refused to face the clouds of sand. A parching thirst oppressed us, which nothing alleviated."
How the prevailing Winds affect the Sand of the Desert. -The prevailing winds in the Sahara are the easterly and westerly, the first blows nine months, the second but three months. This circumstance is intimately connected with the motions and distribution of the sand of the Desert. In the eastern half of the Sahara the sand is more gravelly, and the general cover of sand shallower than in the western half; so that, in travelling towards the west, the depth of the sand and the completeness of the sandy covers increases. This distribution of the sand is probably owing to the easterly wind, which blows so much longer than the westerly, carrying the sand before it from the East Sahara. To the same cause we may refer the less frequent appear ance of rocks, the gradual diminution in magnitude and of
frequency of oases, even their total destruction by blowing sand as we advance westward.*
What is the Geognostical Age of the Sahara?-Many are of opinion that the Sahara must at one time have been the bed of the ocean. The very frequent saline impregnation of the sand, the rolled pebble and sands mixed with seashells at the foot of the southern acclivity of the Atlas and other parts of the Desert, are considered as in favour of this hypothesis. At what period did this great tract rise above ́ the waves of the ocean? This can only be guessed at by an attentive examination of the junctions of the sandstones, limestones, &c. with the bounding primary ranges of the Desert. If they are the same on the south side as on the north or Atlas side, then it would follow that the Desert rose above the sea at the time when the Atlas made its appearance from below; that is, after the deposition of the tertiary rocks, at a period when the earth and its animals and vegetables were nearly the same as at present.
3. Geology of the Region to the South of the Sahara, and to the North of the Great Table-land.-This is the Land of the Negroes, called also Soudan or Nigritia. The high land on the west of this part of Africa is partly accumulated around the sources of the rivers Senegal, Gambia, Rio Grande, and Niger or Joliba. From the sources of the Niger the mountains run eastwards, under the name Kong Mountains, across Africa, when at length they are said to form a junction with the Mountains of the Moon, that range onward and join with the vast alpine land of Abyssinia. Parts of this boundary are very lofty, some mountains of the Kong chain attaining an elevation of 14,000 feet above the level of the sea. From the meager details of travellers in regard to this part of Africa, all we can infer is, that the mountains on the west and along the south of this zone contain primitive rocks of various descriptions, as granite, mica-slate, clay-slate, quartz rock, hornblende rock, limestone, &c. In different parts these rocks seem traversed by augite greenstone or secondary traps. The secondary sandstones and limestones connected with these ranges not having been accurately described, we cannot venture any
*The long continuance of the easterly in comparison of the westerly wind, may explain how it happens that the whole country of Egypt has not ere this been swallowed up by the sand-flood of the Desert.
conjecture as to their geological nature. At Gambia there is only sand; but opposite the town there are islands of red decomposed granite. At Goree the rock is a fine basalt, which takes a regular prismatic form, similar to the Giants' Causeway.*
Vast tracts of flat country, partly rich and cultivated, partly desert and sandy, extend to the eastern limit, including Soudan, of which the great kingdoms are Houssa and Bornou. In the flat and desert regions, salt lakes and natron lakes, and salt and natron springs, are met with. Beds of rock-salt occur in different places, as at Teleg, north of Timbuctoo, half a day's journey from Taudeny. From this place is exported all the salt from Timbuctoo to Jenne, and from that town to Soudan. The salt is there disposed in beds several feet thick: it is mined into large slabs, which are afterward sawn into blocks for the market. These mines form the riches of the country.
African Gold. This continent, as is well known, affords a considerable quantity of gold, which is found in the form of rolled pieces, or in minute grains, named gold dust, in the alluvium of rivers, lakes, valleys, and the wide-spreading sand of the vast Desert. The northern parts of Africa afford but little gold; while in the countries to the south of the Great Desert, there are tracts remarkable for the quantity of gold they contain. Thus the flat country, which extends from the foot of the mountains in which are situated the sources of the Gambia, Senegal, and Niger, has, from an early period, afforded gold. Bambouk, which is situated to the north-west of these mountains, furnishes the greatest part of the gold which is sold on the western coast of Africa, as well as that which is brought to Morocco, Fez, Algiers, Cairo, and Alexandria. The gold, as is often the case, is accompanied with grains of iron ore, probably the magnetic or black iron ore. Gold mines occur to the south of Timbuctoo. The people employed in these mines are Bambarra negroes, who become wealthy, as all the particles of gold under a certain weight (12 mizams) belong to them. Pieces of gold, weighing several ounces, are sometimes found there. The country of Kordofan, to the south-east of the Great Desert, affords a considerable quantity
* Geol. Tr., vol. i., New Series, p. 418.
THOMAS PARK'S ACCOUNT of accra.
of gold. The precious metal found in that country is brought to market by the negroes, in quills of the ostrich and vulture. This territory, it would appear, was known to the ancients, who regarded Ethiopia as a country rich in gold. Sulphur is said to occur in Darfur.
4. Great Table-land of Africa.-Of the table-land itself we know very little, the geological details we are now to lay before our readers being principally illustrative of the mountain-ranges and acclivities that surround this elevated plateau.
Geology of the Coast from Sierra Leone to Cape Negro. -We shall trace the geological phenomena from Sierra Leone to Cape Negro. The hills around Sierra Leone are of granite, or rather of a porphyritic granitic syenite, in which tourmaline crystals occur.* We know nothing whatever of the geology of the Grain Coast and Ivory Coast of Guinea. The Gold Coast is so named from the great trade in gold dust carried on there, which has given rise to many European settlements. We are told that in the interior there are mountains of granite, gneiss, and quartz, and that the gold is collected from the alluvial sands and clays formed from these rocks. Nothing particular is known of the rocks or soils of the Slave Coast.
Our young friend and pupil, Thomas Park, son of the celebrated but unfortunate Mungo Park, possessing the enthusiasm and courage of his father, determined on traversing Africa, with the view of ascertaining the history of his father's fate, at that time in some degree unknown, and also of enlarging our knowledge of its natural history and geography. He was landed by order of government at Accra, on the west coast, in 5° N. The last letter we received from this promising young traveller,-for shortly after the commencement of his journey he perished,-was as follows:-" Accra, 17th September, 1827.-I intend to set off to-morrow morning. I have been, as you know, three months here, during which time I have been principally busy with the study of the Ashantee language. Some time ago I made an excursion of about fifty miles into the interior, by way of experiment, and did not fail to look around me and notice the rocks and other natural produc
* Geol. Tr., vol. i., New Series, p. 418.