The most important group of deserts in the world is that of the Sahara, which extends across the African continent from the shores of the Atlantic to the valley of the Nile. This immense area is more than 3,100 miles from east to west, and is, on an average, more than 600 miles in breadth; it is, in fact, equal in size to two-thirds of Europe. In this region there is only one season, viz., summer, burning and merciless. It is but rarely that rain comes to refresh these regions, on which the solar rays dart vertically down. The mean altitude of the Sahara is estimated at 2,000 feet; but the level of the soil varies singularly in the different districts. To the south of Algeria, the surface of the Chott Mel-R’ir, the remains of an ancient sea, which communicated with the Mediterranean, is at the present time more than 165 feet below the Gulf of Cabes; while to the south and east, the ground rises into plateaus and mountains of sandstone or granite to a height varying from 3,300 to 6,660 feet. In the centre of the Sahara stands the Djebel-Hogger, the sides of which are covered with snow during three months in the year; from December to March, its picturesque defiles are traversed by streams which flow some distance and lose themselves beneath the surrounding plains. This group of lofty mountains is the great landmark which forms the boundary between the eastern deserts, or the Sahara proper, and the group of western deserts, designated under the general name of Sahel. The Sahel is very sandy. Throughout the greater part of its extent the soil is composed of gravel and large-grained sand, which does not give way even under the foot of the camel. Some of the ranges of sand-hills which rise in this desert are chains of small hills, composed of heavy sand which resists the influence of the wind. But in many districts of the Sahel, the arenaceous particles of the soil are fine and small. The trade-winds which pass over the desert distribute these sandy masses into long waves similar to those of the ocean, and here and there raise them into movable sand-hills, which overwhelm all the oases which lie across their path. Traveling toward the southwest, in which direction they are driven by the wind, the sands reach the northern shores of the Niger and Senegal at many points of their course, and by their incessant deposits gradually drive the waters of these rivers toward the south. To the west, the sand of the desert encroaches also upon the ocean. Off the coast which stretches between Cape Bojador and Cape Blanco--pointed out from afar by the highest dunes in the world--a line of sand-banks extends far out into the sea. A current of sand is, therefore, constantly passing across the desert from northeast to southwest. The débris of rocks in a state of decomposition, and the particles brought to the coast of the Gulf of Cabes by the tide, which is very powerful at this point, are driven before the wind into the plains of