Page:Africa by Élisée Reclus, Volume 3.djvu/156

 124 WEST AFRICA. This highland system, which begins on the Senegal in the Bondu district, does not appear to develop a regular chain till it approaches the great bend of the Gambia. It runs mainly in the direction from north to south, with a slight eastern deflection, for a total length of about 180 miles. But beyond the sources of the Senegalese Bafing the' chain is continued by other still unexplored mountains south-westwards to the hills, from 3,500 to 4,500 feet high, which command the sources of the Niger. In Senegambia the most abrupt slope faces eastwards in the direction of the Gambia and Faleme, and a large part of the system consists of baowals, or slightly rolling plateaux strewn with scattered boulders and broken by steep escarpments. Geologically, Futa- Jallon forms a nucleus of crystalline rocks encircled by more recent formations, and by most travellers described as consisting of granites, gneiss, and "primary sandstones." Northwards and north-eastwards this crystalline and schistose mass is continued by other parallel ridges, such as the Tambaura of Bambuk, where almost inaccessible rocky walls rise abruptly about the surrounding verdant plains, and the heights of Kenieba, affecting the form of truncated cones. The secondary ridges are intersected by river valleys, whoso sands and clays contain particles of gold washed from the primitive rocks. Through this auriferous alluvia the headstreams of the Senegal have excavated their convergent beds. Between the Bafing and the Bakhoy, the two main forks of the Senegal, the hills running parallel to the Niger consist of horizontally stratified sandstones, above which crop out granite, hornblende, quartz, and feldspar blocks of fantastic shape. Even north of the Senegal as far as the sands of the desert, the lines of hills and terraces consist of sandstones dating from the same epoch. In Kaarta the Saharian cliffs, whence flow the intermittent a^uents of the Senegal, have a mean height of from 1,000 to 1,070 feet, and the hills are h6re formed of bluish slaty schists overlain with deposits of laterite. Farther west the heights are more regularly disposed in chains running mostly in the direction from north-east to south-west. The surface looks as if it had been furrowed by a gigantic plough, leaving between the trenches parallel Tidges with their steep side facing east and sloping gently westwards. The western crest of Halip Anaghim, forming the north-west limit of the Senegal basin, rises to a height of 1,350 feet. West of the Senegambian gneiss and schists follows a deposit of ferruginous sandstones or laterites, an ochreous mass formed by the disintegration of the older rocks, and occupying all the Senegambian seaboard, except where the streams and tides have deposited their alluvia. Towards the west these ochreous sandstones contain a continually increasing proportion of iron, and in many places the ground looks as if it were covered with ferruginous refuse like the neighbourhood of a smelting furnace. The Senegambian Seaboard. The Senegambian seaboard is disposed in three distinct geological sections, the first extending from Cape Blanco to Cape Verd, the second from Cape Yerd to Cape Roxo (" Eed "), the third thence to the island of Sherbro. Taken as a whole