Now, my young friend, what do you understand by a desert? I think I can guess. When you read or hear of a desert, you think of a vast level plain, covered with sand, and without trees, or grass, or _any_ kind of vegetation. You think, also, of this sand being blown about in thick yellow clouds, and no water to be met with in any direction. This is your idea of a desert, is it not? Well, it is not altogether the correct one. It is true that in almost every desert there are these sandy plains, yet are there other parts of its surface of a far different character, equally deserving the name of _desert_. Although the interior of the great Saara has not been fully explored, enough is known of it to prove that it contains large tracts of mountainous and hilly country, with rocks and valleys, lakes, rivers, and springs. There are, also, fertile spots, at wide distances from each other, covered with trees, and shrubs, and beautiful vegetation. Some of these spots are small, while others are of large extent, and inhabited by independent tribes, and even whole kingdoms of people. A fertile tract of this kind is called an oasis; and, by looking at your map, you will perceive that there are many oases in the Saara of Africa. Of a similar character is the Great American Desert; but its surface is still more varied with what may be termed "geographical features." There are plains--some of them more than a hundred miles wide--where you can see nothing but white sand, often drifting about on the wind, and here and there thrown into long ridges such as those made by a snowstorm. There are other plains, equally large, where no sand appears, but brown barren earth utterly destitute of vegetation. There are others, again, on which grows a stunted shrub with leaves of a pale silvery colour. In some places it grows so thickly, interlocking its twisted and knotty branches, that a horseman can hardly ride through among them. This shrub is the _artemisia_--a species of wild sage or wormwood,--and the plains upon which it grows are called by the hunters, who cross them, the _sage prairies_. Other plains are met with that present a black aspect to the traveller. These are covered with lava, that at some distant period of time has been vomited forth from volcanic mountains, and now lies frozen up, and broken into small fragments like the stones upon a new-made road. Still other plains present themselves in the American Desert. Some are white, as if snow had fallen freshly upon them, and yet it is not snow, but salt! Yes; pure white salt-- covering the ground six inches deep, and for fifty miles in every direction! Others, again, have a similar appearance; but instead of salt, you find the substance which covers them to be soda--a beautiful efflorescence of soda!