Sometimes the desert is a wilderness of broken, stony hills covered by angular pieces of shivered rock. In other places the soil is hard, and is everywhere covered by pebbles or shingle. Often it is a mere waste of sand blown into downs and hillocks which look sometimes like the sand dunes by the coast, and elsewhere like the waves of the sea. One finds valleys in the desert quite like ordinary ones in shape, but instead of water there is only sand in sweeping curves and hollows, like the snow-wreaths and drifts in a highland glen. Rocks stand out of this, but their projecting faces are polished smooth and glittering or deeply cut by the flinty particles scraping over them continually in storms and hurricanes. The traveller on camel-back, where his waist has to act as a sort of universal joint giving to every unexpected jolt and wrench of his rough-paced mount, suffers from the heat, for nowhere else in the world are there such high temperatures. He suffers from thirst, and still more from the dust which fills eyes, mouth, nostrils, and ears. Yet the dry pure air is most exhilarating. In the evening there is a feast for the eyes in the glorious _afterglow_ when the sun has just set. The light from below the horizon produces an ever-changing, indescribable play of colour from violet to salmon pink and through the most delicate shades of yellow, blue, and rose, until everything fades and there reigns only the mysterious silence of the beautiful starlit night. No wonder the air is dry and pure, for rain only falls on perhaps eight days in the year in some places (Ghardiaia). Yet plants manage to exist even where there is only about seven inches of rain annually. But this seems still more extraordinary if one remembers that sand may be almost glowing hot during the day, whilst in winter it may be, at night, cooled below the freezing-point. Yet a desert absolutely bare of plants is an exceedingly rare phenomenon. Such do occur. Darwin speaks of "an undulating country, a complete and utter desert." This is not very far from Iquique in South America. "The road was strewed with the bones and dried skins of the many beasts of burden which had perished upon it from fatigue. Excepting the _Vultur aura_, which preys on the carcases, I saw neither bird, quadruped, reptile, nor insect. On the coast mountains, at the height of about 2000 feet, where, during this season, the clouds generally hang, a very few cacti were growing in the clefts of rock; and the loose sand was strewed over with a lichen which lies on the surface quite unattached. ... In some parts it was in sufficient quantity to tinge the sand, as seen from a distance, of a pale yellowish colour. Farther inland, during the whole ride of fourteen leagues, I saw only one other vegetable production, and that was a most minute yellow lichen, growing on the bones of the dead mules."[61] [61] Darwin, _Naturalist's Voyage round the World in the Beagle_, p. 387.