line of volcanic action continued by the islands to the south-west, has a height of , while Clarence Peak, in Fernando Po, the first of the line of islands, rises to over . Towards the extreme west the Futa Jallon highlands form an important diverging point of rivers, but beyond this, as far as the Atlas chain, the elevated rim of the continent is almost wanting. Plains Much of Africa is made up of plains of the pediplain and etchplain type often occurring as steps. The etchplains are commonly associated with laterite soil and inselbergs. Inselberg-dotted plains are common in Africa including Tanzania, the Anti-Atlas of Morocco, Namibia, and the interior of Angola. One of the most wideaspread plain is the African Surface, a composite etchplain occurring across much of the continent. The area between the east and west coast highlands, which north of 17° N is mainly desert, is divided into separate basins by other bands of high ground, one of which runs nearly centrally through North Africa in a line corresponding roughly with the curved axis of the continent as a whole. The best marked of the basins so formed (the Congo Basin) occupies a circular area bisected by the equator, once probably the site of an inland sea. Running along the south of desert is the plains region known as the Sahel. The arid region, the Sahara — the largest hot desert in the world, covering — extends from the Atlantic to the Red Sea. Though generally of slight elevation, it contains mountain ranges with peaks rising to Bordered N.W. by the Atlas range, to the northeast a rocky plateau separates it from the Mediterranean; this plateau gives place at the extreme east to the delta of the Nile. That river (see below) pierces the desert without modifying its character. The Atlas range, the north-westerly part of the continent, between its seaward and landward heights encloses elevated steppes in places broad. From the inner slopes of the plateau numerous wadis take a direction towards the Sahara. The greater part of that now desert region is, indeed, furrowed by old water-channels. Mountains The mountains are an exception to Africa's general landscape. Geographers came up with the idea of "high Africa" and "low Africa" to help distinguish the difference in Geography; "high Africa" extending from Ethiopia down south to South Africa and the Cape of Good Hope while "low