The geological structure of the mountains of Africa, especially of South Africa, appears to be quite uniform. They have a nucleus of granite which often appears at the surface and forms the predominating rock, but in the greater proportion of the mountains, perhaps, the granite is overlain by vast masses of sandstone, easily distinguished by the numerous pebbles of quartz which are embedded in it. The summit, when composed of granite, is usually round and smooth, but when composed of the quartzose sandstone is often perfectly flat. Of this Table Mount, in South Africa, is a notable illustration. The thickness of this stratum of sandstone is sometimes not less than 2,000 feet. Such is the case in the Karoo mountains of Cape Colony. When thus appearing, it may be seen forming steep, mural faces, resembling masonry, or exhibiting a series of salient angles and indentations as sharp, regular, and well-defined as if they had been chiselled. With the granite are often associated primitive schists, the decomposition of which seems to have furnished the chief ingredients of the thin, barren clay which forms the characteristic covering of so much of the South African mountains. In some places, more recent formations appear, and limestone is seen piercing the surface. The geological constitution of the Atlas Mountains, in northwestern Africa, presents old limestone alternating with a schist, often passing to a well-characterized micaceous schist, or gneiss, the stratification of which is exceedingly irregular. Volcanic rocks have here been found in small quantities. There are veins of copper, iron, and lead. In Egypt we find the alluvial soil a scarcely less interesting object of study than the rocks upon which it rests. These are limestone, sandstone, and granite, the latter of which, in Upper Egypt, often rises 1,000 feet above the level of the Nile. Not many years ago were discovered about 100 miles east of the Nile, and in 28 deg. 4 min. of north latitude the splendid ruins of the ancient Alabastropolis, which once derived wealth from its quarries of alabaster. Farther south are the ancient quarries of jasper, porphyry, and verd antique. The emerald mines of Zebarah lay near the Red Sea. The Atlas range in Algeria is better known than elsewhere. It is as described above, but at Calle there are distinct traces of ancient volcanoes. Iron, copper, gypsum, and lead are found in considerable quantities. Cinnabar is found in small quantities. Salt and thermal springs abound in many parts of Algeria, amethysts in Morocco, slates in Senegambia, and iron in Liberia, Guinea, the Desert of Sahara and many other parts of Africa.