CHAPTER XVII. Geology of Africa.* AFRICA is distinguished from the other continents by its nearly insular form, being connected with Asia merely by an inconsiderable neck of land or isthmus, viz. that of Suez. It extends from the equator to about the average latitude of 35° north, and also to the same degree of latitude south. The greatest length from north to south is from Cape Serrat in Algiers, in lat. 37° 18′ N., to Cape Laguillas, in lat. 34° 55′ S.; and the greatest breadth from Cape Verde, in long. 17° 31′ W., to Cape Guardafui, in long. 51° 15' E. The northern portion of this continent is fully twice the size of the southern portion, and may be considered as about equal to South America; while the southern half is contracted to half the breadth of the northern part, and is nearly about the size of New Holland. The shape of the corresponding coasts of Africa and America would induce us to infer that the two continents of Africa and America were once united,—the projecting or salient part of the former fitting exactly to the Gulf of Mexico; and the bulging part of South America, about Paraiba and Pernambuco, being According to some authors, the name, Africa, is derived from a neg. and frigus, cold; while others trace it from a small Carthaginian district, named Frigi-A-frikc-a. nea. about the size and shape to fill up the Gulf of GuiThis great continent has but comparatively few gulfs, bays, arms of the sea, and promontories; and hence, notwithstanding its nearly insular form, its extent of coast is much less in proportion to its area than in other quarters of the globe. The condition of man, the distribution of the lower animals and plants, even the climate of Africa, are intimately connected with this limited extent of coast. On viewing Africa from its northern boundary on the shores of the Mediterranean, to its southern boundary at the Cape of Good Hope, the following natural divisions or regions present themselves to our attention: 1. The northern region formed by the Atlas range of mountains, hills, and plains, that extend from the coast of the Atlantic to the Gulf of the Syrtis,-and by the range of fertile hills and dales, and valleys mixed with deserts, in which are some insulated spots of verdure, known under the name oases,* that extend from the termination of the Atlas to Egypt.t 2. The eastern region formed by Egypt, Abyssinia, Darfur, &c. 3. The Desert region, which is the flat, compa The word Oasis is Egyptian, and synonymous with Auasis and Hyasis, (Strabo, Ixxiii., p. Alm. 1140). Abulfeda names the Oasis Al Wahat. In latter times the Cæsars banished criminals to the Oases. They were sentenced to expiate their crimes on the islands of the Sandy Sea, as the Spaniards and English send their criminals to the Malouin Islands and New Holland. The latter could more easily escape by the ocean, than the former across the surrounding deserts. + The Atlas of Homer and Hesiod, according to Bory St Vincent, is the Peak of Teneriffe; the Atlas of the Greek and Roman geographers, the African Atlas range of mountains. ratively low tract of generally desert country, of which the principal portion is the Great Desert of Sahara, which lies between the 29th and 16th parallels, or about 780 miles in breadth, and extending across the continent from the Atlantic to the borders of Nubia. 4. The Region of Soudan, Nigritia, or the Country of the Negroes, extending in a belt across the continent as far as Abyssinia, and from the 16th to the 5th parallel, or about 600 miles in breadth. It is a rich and fertile region, yielding, with little labour, all the valuable productions of the tropical countries. 5. Great Table-Land of Africa, or High Africa. -This, in all probability, very interesting part of Africa extends from the zone of Nigritia to the Cape of Good Hope. It appears to contain a lofty and extensive table-land, from which acclivities, supporting ranges of mountains, decline on the east and south towards the Indian Ocean; on the west to the Atlantic; and on the north to the Country of Soudan or Nigritia. Unfortunately the whole of this great region, with exception of the Cape of Good Hope and the Portuguese settlements on the east and west coasts, between which they are said to keep up a communication, is unknown to us; so that there still remains a tract of country, at the least 30 degrees of latitude by 25 of longitude, or about 2,600,000 square geographical miles, of which nothing whatever is known. Now that the thirst for Arctic discovery has been quenched, and the public feeling has set strongly against expeditions to Central Af rica, we trust that our government will be the first to engage in the exploration of the great table-land of Southern Africa. Having premised this short account of the gene ral features of Africa, we shall now state what is known of its geology and mineralogy, following in our account the great natural divisions already pointed out. 1. Geology of the Atlas, or Northern Region of Africa.*—The northern division of Africa is principally characterised by the Atlas chain of mountainranges, on some of the loftiest points of which there is perpetual snow, which gives them a height of 12,000 to 13,000 feet above the level of the sea. In it there are rocks of the primitive class, as granite, gneiss, mica-slate, and clay-slate. Copper and lead mines, said to occur in the primitive parts of the range, were worked by the ancients in Morocco and Algiers, but are at present neglected; and the same is also the case with the antimony and tin (?) said to have been discovered in these mountains. In Tunis, rock-crystals, graphite, or black-lead, and also iron and galena, are met with in the same kinds of rock. Although in extensive mountain-ranges the older rocks, or those of the primitive class, generally predominate, such, according to travellers, is not the case with the Atlas, where the most extensive deposites are of a calcareous nature. This calcareous formation consists principally of secondary limestones, associated with deposites of sandstone. The limestone abounds with organic remains, as of shells, corals, and even fishes; and is said to be referable to the various limestones extending from the lias, or even the magnesian limestone, to chalk inclusive. Hence in this limestone range there are magnesian limestones, oolite limestones, lias limestones, Jura * The Egyptian, Abyssinian, and bordering African districts, will be considered in one of the succeeding volumes of this work. limestones, and soft limestones resembling some kinds of chalk. Resting upon these limestones, or where they are wanting, as is the case at Algiers, there are deposites of tertiary rocks; these are marly clays and limestones, with organic remains resembling those met with in the tertiary deposites on the north shore of the Mediterranean. Salt springs and gypsum are mentioned as occurring in different parts of the range. These may be connected either with the secondary or tertiary, or with both classes of rocks. Trap-rocks, of a modern date, also make their appearance among the rocks of the northern African zone. The most extensive display of these Plutonian masses is in the limestone in some districts to the south of Tripoli, where these rocks alter the position and change the characters of the limestone. Age of the Atlas Mountains.-It is conjectured, by some geologists, that the great ranges of mountains of the earth have risen from below, through rents in previously existing strata, and not all at once, but at different times; and further, that all mountain ranges having the same general direction have made their appearance from below at the same time. Thus the Pyrenees and Apennines, the mountains of Dalmatia and Croatia, and the Carpathians, which belong to the same system,— as may be deduced from the descriptions given of them by various geologists,—are all disposed parallel to an arc of a great circle, which passes through Natchez and the mouth of the Persian Gulf. Thus, whatever may have been the cause, the mountains in Europe, which have issued from the earth at the same period, form chains at the surface of the globe, that is to say, longitudinal projections, |