With a few rare exceptions they lie beyond the green plains on the threshold of the solitudes.

And against the changeless, rose-coloured background of these cliffs of the desert, which follow you during the whole of this tranquil navigation of some 600 miles, are to be seen only the humble towns and villages of to-day, which have the neutral colour of the ground.

Some openwork minarets dominate them--white spots above the prevailing dullness.

Clouds of pigeons whirl round in the neighbourhood.

And amongst the little houses, which are only cubes of mud, baked in the sun, the palm-trees of Africa, either singly or in mighty clusters, rise superbly and cast on these little habitations the shade of their palms which sway in the wind.