The fertile highlands of Kenya, of which the great mountain of that name is the northern outpost, are divided from those of Abyssinia by a great desert nearly 400 miles wide. This tract is a gneiss peneplain of great age, whose degradation, without any regional interruption, has continued since Palaeozoic times. At the present date its undulations are scarcely noticeable, and its ill-supplied rivers do not often reach the coast. Such is the Uaso Nyero, which, rising from the north-western slopes of Mount Kenya, and running spasmodically for 200 miles eastwards through this desert, finally dies away in the great swamp called Lorian, which shifts its position up or down the valley in accordance with the seasons. The Tana River reaches the sea, and its valley also has the appearance of great antiquity; the long slopes down to the valley from its sides are hardly perceptible, and for two-thirds of its 400 mile course the river meanders sluggishly from one side of its valley to the other.