Fortunately, all parts of the Great Desert are not so dangerous to life. A large portion, if not the greater portion of it, consists of barren but firm soil. This soil is composed of hardened sand, sandstone, and granite. Quartz rock is sometimes found in small quantities. This portion of the Sahara often rises into ridges, or small hills. These chains of hills are often fully as desolate and bare as the plain from which they rise.
Many of the plains of the Sahara vary from the uniform type. Sometimes on one of them the ground is covered with stones or loose boulders. These are fully as fatiguing to the traveler as the loose, drifting sand. Often these plains are rent by deep chasms. Frequently they are hollowed into great basins.
Into the plains which contain these chasms the rain descends from the gullies of the Atlas mountain system. This sometimes forms streams, which the thirsty sands swallow, or which are quickly evaporated by the burning rays of the sun.
The deeper basins of the Sahara are often of considerable extent. Some of them contain valuable salt deposits.
There are vast tracts, too, of sterile sand, where not even the smallest of plants will take root. In the absence of plant life, animals disappear, and for days the weary traveler may pursue his journey and meet neither beast, bird, nor insect to break the monotony of the dreary waste of sand.
The description of the Sahara by one author is most realistic. "Nowhere are the transitions of light and shade more abrupt than in the desert, for nowhere is the atmosphere more thoroughly free of all vapors. The sun pours a dazzling light on the ground, so that every object stands forth with wonderful clearness, while all that remains in the shade is sharply defined, and appears like a dark spot in the surrounding glare.
"The stillness of these wastes is sometimes awfully interrupted by the loud voice of the simoon. The crystal transparency of the sky is veiled with a hazy dimness. The wind rises, and blows in intermittent gusts, like the laborious breathing of a feverish patient.
"Gradually the convulsions of the storm grow more violent and frequent; and although the sun is unable to pierce the thick dust clouds, and the shadow of the traveler is scarcely visible on the ground, yet so suffocating is the heat, that it seems to him as if the fiercest rays of the sun were scorching his brain.
"The dun atmosphere gradually changes to a leaden blackness, the wind becomes constant, and even the camels stretch themselves upon the ground and turn their backs to the whirling sand storm.
"At night the darkness is complete; no light or fire burns in the tents, which are hardly able to resist the gusts of the simoon. Silence reigns throughout the whole caravan, yet no one sleeps; the bark of the jackal or the howl of the hyena alone sounds dismally, from time to time, through the loud roaring of the storm."