8 A TROPICAL DEPENDENCY

which they need. These mountains have been as the stronghold of civilisation to the coast. Behind them on the southern slopes there is a belt of land on which the date palm flourishes, salt mines abound, and flocks and herds can find subsistence. In this belt there are even

spots of great fertility, and there are parts in which it widens, spreading with fertile promontories into the desert.

But in its nature this southern face of the hills, known to the ancients as the Land of Dates, is but an offshoot of

the coast strip.

It merges soon into the deserts of the rainless zone which form the second great section of North Africa. From the Atlantic coast to the Nile these deserts, under different names, succeed each other across the continent in a broad belt of desolation. Upon the map they cover an area of between ten and fifteen degrees of latitude.

At their narrowest parts the caravans which traverse them count upon a march of fifty days. They are in part composed of drifting sand, through which only long practised local guides can find their way ; they are prac-

tically waterless, and it is of course only in places where springs are known to exist that the passage of them is possible. Marmol, a Spanish writer of the sixteenth century, gives an interesting description of how these

wells were preserved in his day. ** They are," he says,

"walled inside with camels' bones for want of stones, and they are also covered with camels' skins lest the shifting sands should blow over them and fill them up. The

natural consequence is that even when there the wells are often hidden, and the traveller may die of thirst within

a few feet of water."

With the hot sands of the deserts the continent passes into the tropics, and here again a natural barrier marks the third great division of North Africa. A straight line drawn upon the seventeenth parallel of latitude will mark the edge of the zone of summer rains. Slightly to the south of it may be traced the great water-belt formed by the courses of the Senegal, the Niger, the Benue, the